Substrates with discretely immobilized silver nanoparticles for ultrasensitive detection of anions in water using surface-enhanced Raman scattering.
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.
Tan
Siliu
S
Department of Chemical, Biomedical, and Materials Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, USA.
Erol
Melek
M
Sukhishvili
Svetlana
S
Du
Henry
H
D000838:Anions / Q000737:chemistry; D003486:Cyanides / Q000737:chemistry; D005898:Glass / Q000737:chemistry; D053768:Metal Nanoparticles / Q000737:chemistry*; D012834:Silver / Q000737:chemistry*; D013053:Spectrophotometry; D013059:Spectrum Analysis, Raman; D013457:Sulfur Compounds / Q000737:chemistry
D000838:Anions; D003486:Cyanides; D013457:Sulfur Compounds; D012834:Silver