title: 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.
authors.affiliation: Department of Chemistry, Center for Optical Materials Science and Engineering and Environmental Toxicology Program, Clemson University, South Carolina 29634-0973, United States.
authors.forename: Fenglin
authors.initials: F
authors.lastname: Wang
authors.forename: Ryan G
authors.initials: RG
authors.lastname: Widejko
authors.forename: Zhiqiang
authors.initials: Z
authors.lastname: Yang
authors.forename: KhanhVan T
authors.initials: KT
authors.lastname: Nguyen
authors.forename: Hongyu
authors.initials: H
authors.lastname: Chen
authors.forename: Lawrence P
authors.initials: LP
authors.lastname: Fernando
authors.forename: Kenneth A
authors.initials: KA
authors.lastname: Christensen
authors.forename: Jeffrey N
authors.initials: JN
authors.lastname: Anker
mesh_terms: D000818:Animals; D001565:Benzoates / Q000032:analysis*; D002417:Cattle; D002460:Cell Line; D006046:Gold / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D053768:Metal Nanoparticles / Q000737:chemistry*; D051379:Mice; D012710:Serum Albumin, Bovine / Q000737:chemistry / Q000378:metabolism; D012822:Silicon Dioxide / Q000737:chemistry*; D012834:Silver / Q000737:chemistry*; D013059:Spectrum Analysis, Raman; D013438:Sulfhydryl Compounds / Q000032:analysis*; D013499:Surface Properties
chemical_list: D001565:Benzoates; D013438:Sulfhydryl Compounds; C013594:4-mercaptobenzoate; D012710:Serum Albumin, Bovine; D012834:Silver; D006046:Gold; D012822:Silicon Dioxide