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.

A full‐length accumulative photonic crystal fiber (PCF) that is surface‐enhanced Raman scattering (SERS)‐active is achieved by immobilizing Ag nanoparticles inside the air channels of solid‐core and hollow‐core PCFs using a polyelectrolyte‐mediated process. Raman gain in PCFs prevails with coverage density below 0.5 nanoparticle µm−2. Light attenuation dominates, however, at a higher density. Controlled coverage density and uniformity of nanoparticles is the key to the exploitation of the length benefit of the PCF platform.

Positively charged silver nanoparticles, Ag [+], obtained by UV-assisted reduction of silver nitrate using branched poly(ethyleneimine) (BPEI) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) solutions as reducing agents, were immobilized on glass surfaces to produce substrates active in surface-enhanced Raman scattering (SERS). Negatively charged silver nanoparticles, Ag [-], synthesized via a modified citrate reduction method, were also investigated for comparison. At a sparse surface coverage of 30 nanoparticles/microm(2), substrates with immobilized Ag [+] showed increasing SERS sensitivity to a variety of anions in water in the order SO(4)(2-) < CN(-) < SCN(-) approximately ClO(4)(-), with corresponding binding constants of 10(5), 3.3 x 10(5), and 10(7) (for both SCN- and ClO(4)(-)) M(-1), respectively. This order followed the Hofmeister series of anion binding in water. Significantly, substrates with Ag [+] allowed limit of detection values of 8.0 x 10(-8) M (8 ppb) and 2.7 x 10(-7) M (7 ppb) for environmentally relevant perchlorate (ClO(4)(-)) and cyanide (CN(-)) anions, respectively. In contrast, substrates with immobilized Ag [-], even upon subsequent modification by a monolayer of BPEI for positive surface charge of the nanoparticles, showed a drastically lower sensitivity to these anions. The high sensitivity of substrates with Ag [+] for anion detection can be attributed to the presence of two types of functional groups, amino and amide, on the nanoparticle surface resulting from UV-assisted fragmentation of BPEI chains. Both amino and amide provide strong binding of anions with Ag [+] nanoparticles due to the synergistic effect through a combination of electrostatic, hydrogen bonding, and dispersive interactions.

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