Substrates with Discretely Immobilized Silver Nanoparticles for Ultrasensitive Detection of Anions in Water Using Surface-Enhanced Raman Scattering

Abstract

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.

Chemical Optimization for Simultaneous Voltammetric Detection of Molybdenum and Silver Nanoparticles in Aqueous Buffer Solutions

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SERS detection of indirect viral DNA capture using colloidal gold and methylene blue as a Raman label

“…When using colloidal nanoparticles as the SERS substrate, the overall SERS spectrum for an analyte can arise from a composite of interactions, some of which include the formation of charge-transfer states for adsorbed molecules on metal surfaces, the potentiation of analyte signaling by small molecules and nanoparticle aggregation (Aroca et al, 2005). Anions, and halides in particular, have been shown to both enhance the SERS spectrum associated with analyte detection and contribute unique spectral peak signatures by displacing citrate from citrate-stabilized nanoparticles in accordance with their equilibrium binding constants with Au + or Ag + (Bell and Sirimuthu, 2005;Maryuyama and Futamata, 2007;Tan et al, 2008). Although less well understood, cation-induced enhancements of SERS responses have also been reported (i.e.…”

Graphene improved electrochemical property in self-healing multilayer polyelectrolyte film

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