We quantitatively studied, using X-ray photoelectron spectroscopy (XPS), oxidation of substrate-immobilized silver nanoparticles (Ag NPs) in a wide range of conditions, including exposure to ambient air and controlled ozone environment under UV irradiation, and we correlated the degree of silver oxidation with surface-enhanced Raman scattering (SERS) enhancement factors (EFs). The SERS activity of pristine and oxidized Ag NPs was assessed by use of trans-1,2-bis(4-pyridyl)ethylene (BPE) and sodium thiocynate as model analytes at the excitation wavelength of 532 nm. Our study showed that the exposure of Ag NPs to parts per million (ppm) level concentrations of ozone led to the formation of Ag(2)O and orders of magnitude reduction in SERS EFs. Such an adverse effect was also notable upon exposure of Ag NPs under ambient conditions where ozone existed at parts per billion (ppb) level. The correlated XPS and SERS studies suggested that formation of just a submonolayer of Ag(2)O was sufficient to decrease markedly the SERS EF of Ag NPs. In addition, studies of changes in plasmon absorption bands pointed to the chemical enhancement as a major reason for deterioration of SERS signals when substrates were pre-exposed to ambient air, and to a combination of changes in chemical and electromagnetic enhancements in the case of substrate pre-exposure to elevated ozone concentrations. Finally, we also found UV irradiation and ozone had a synergistic effect on silver oxidation and thus a detrimental effect on SERS enhancement of Ag NPs and that such oxidation effects were analyte-dependent, as a result of inherent differences in chemical enhancements and molecular binding affinities for various analytes.

Oxidation of the Ag nanoparticle surface has a dramatic effect on the adsorption, orientation, and SERS detection limit of nitroaromatic molecules in aqueous solutions. Ultrasensitive SERS detection of p-nitrophenol can be achieved when oxidation of surface-immobilized Ag nanoparticles is inhibited by replacing the oxygen dissolved in water with argon gas. The presence of silver oxide at the nanoparticle surface hinders charge transfer between the aromatic ring and the underlying Ag metal surface and drastically decreases the overall detection sensitivity.

Photosensitizer, protoporphyrin IX (PpIX), was conjugated with Au nanoparticles (Au NPs) of 19, 66, and 106 nm diameter to study the size-dependent enhancement of reactive oxygen species (ROS) formation enabled by Au NPs. The ROS enhancement ratio is determined to be 1:2.56:4.72 in order of increasing Au NP size, in general agreement with theoretically calculated field enhancement to the fourth power. The convergence of the experimental and simulated results suggests that Au NP-enhanced and size-dependent ROS formation can be attributed directly to the localized electromagnetic field as a result of surface plasmonic resonance of Au NPs under light irradiation. In vitro study on the ROS formation enabled by PpIX-conjugated Au NPs in human breast cancer cells (MDA-MB-231) revealed the similar size-dependent enhancement of intracellular ROS formation, while the enhancement greatly depended on cellular uptake of Au NPs. Cellular photodynamic therapy revealed that cell destruction significantly increased in the presence of Au NPs. Compared to the untreated control (0% destruction), 22.6% cell destruction was seen in the PpIX alone group and more than 50% cell destruction was obtained for all PpIX-conjugated Au NPs. The 66 nm Au NPs yielded the highest cell destruction, consistent with the highest cellular uptake and highest ROS formation. Clearly, the complex cellular environment, size-dependent cellular uptake of Au NPs, and ROS generations are vital contributors to the overall cellular PDT efficacy.

Branched polyethyleneimine (BPEI) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) were used collaboratively to reduce silver nitrate under UV irradiation for the synthesis of positively charged silver nanoparticles. The effects of molar ratio of the ingredients and the molecular weight of BPEI on the particle size and distribution were investigated. The mechanism for the reduction of Ag+ ions in the BPEI/HEPES mixtures entails oxidative cleavage of BPEI chains that results in the formation of positively charged BPEI fragments enriched with amide groups as well as in the production of formaldehyde, which serves as a reducing agent for Ag+ ions. The resultant silver nanoparticles are positively charged due to protonation of surface amino groups. Importantly, these positively charged Ag nanoparticles demonstrate superior SERS activity over negatively charged citrate reduced Ag nanoparticles for the detection of thiocyanate and perchlorate ions; therefore, they are promising candidates for sensing and detection of a variety of negatively charged analytes in aqueous solutions using surface-enhanced Raman spectroscopy (SERS).

We report a study on polymer-mediated immobilization of non-aggregated Ag nanoparticles on planar glass substrates and the resultant surface-enhanced Raman scattering (SERS) activity using Rhodamine 6G (R6G) as a model molecule. Ag colloidal solution with an average particle diameter of 70 nm was prepared by citrate reduction of AgNO 3 using the Lee-Meisel method, and subsequent fractionation by filtration. A self-assembled polyallylamine hydrochloride (PAH) monolayer was employed as the intermediate polymer layer. We have shown that the coverage density of Ag nanoparticles on the glass substrates correlates with the amount of adsorbed PAH. This parameter can be easily controlled by varying the pH and ionic strength during polymer deposition. The highest coverage density was obtained for the polymer deposition from buffer solutions at pH 9.0, which additionally contained 0.25 M NaCl. The SERS-active substrates were robust and stable in 0.5 M NaCl solutions, as well as under extreme acidic and basic conditions. The glass substrates with immobilized non-aggregated Ag nanoparticles exhibited SERS enhancement and provided in situ detection sensitivity of R6G at 5 ppt level, with estimated surface coverage of two to four R6G molecules per silver particle. We found that adsorption of R6G in the presence of N-(2-hydroxyethyl)piperazine-N -(2-ethanesulfonic acid) (HEPES) buffer resulted in complete inhibition of photodecomposition of adsorbed R6G molecules. Studies of the effect of sodium chloride on the SERS activity of the glass substrates with individually attached nanoparticles showed that chloride anions resulted in a two to threefold increase in SERS intensity. Our approach has enabled the isolation of chloride-activated SERS enhancement from contributions arising from nanoparticle aggregation. The observed SERS enhancement in salt solutions results from coadsorption of chloride ions by the silver surface, and manifests itself in changing of orientation of adsorbed R6G molecules. The observed enhancement was reversed by exposing the substrate to 514-nm laser radiation, which resulted in the cleavage of Ag-Cl bonds.