Indian Journal of Chemistry Vol. 54A, March 2015, pp. 316-323 Surface embedded enhancement of fluorescence of coumarinyl-azo-imidazolium stabilized gold nanoparticles Chiranjit Patra, Himanish Roy, Chandana Sen, Ambikesh Mahapatra & Chittaranjan Sinha* Department of Chemistry, Jadavpur University, Kolkata 700 032, India Email: c_r_sinha@yahoo.com Received 20 October 2014; revised and accepted 23 February 2015 Coumarinyl-azo-imidazole (CAI-H) undergoes double alkylation to yield 1, 3-dialkyl-2-(coumarinyl-6-azo)imidazolium bromide (CAI-(C n H 2n+1 ) 2 + Br - ) of long chain alkyl groups –C n H 2n+1 (n = 6, 9, 12, 15, 18) attached to imidazolyl motif. These compounds are characterized by spectroscopic data (FT-IR, UV-vis and 1 H NMR). The size of the gold nanoparticles stabilized with tetraoctylammonium bromide (avg size 20.5 nm) is reduced to 2–10 nm upon surface implantation of CAI-(C n H 2n+1 ) 2 + ; these have been characterized by FESEM/TEM. The spectroscopic investigation has established the stability of gold nanoparticles. The fluorescence emissivity of these nanoparticles is enhanced 10–25 folds as compared to free CAI-(C n H 2n+1 ) 2 + Br – , which may be due to coupling of plasmonic band with emission of fluorescent dye molecules and the reduction of PET (Coumarin*→ Imidazolium) and vibrational relaxation of imidazolium coating on the gold nanoparticles surface. Keywords: Nanoparticles, Gold nanoparticles, Fluorescence, Quantum yield, Coumarins, 1, 3-Dialkyl-2-(coumarinyl-6-azo) imidazolium bromide Gold nanoparticles are promising agents for different field of activity such as electronics, photochemical, biomedicine and chemistry 1-7 . The size, shape and surface environment of gold nanoparticles influence the surface plasmon resonance energy and symmetry which have been modulated by surface implantation of specific molecular units 8 . These properties of the gold nanoparticles have attracted the attention of both theoretical and experimental researchers and have motivated the synthesis of gold nanoparticles in different environments following different techniques and methods. Ionic liquids such as imidazolium based solvents are very useful reagents for the synthesis of nanoparticles, through non-conventional activation routes. Imidazolium capped stable gold nanoparticles are easily functionalized on the imidazole motif and may serve as the communication antenna to the nanoparticles surface 9,10 . Fluorophore in the vicinity of silver or gold nanostructures shows enhanced emission efficiency which is due to the increased electromagnetic field around the metal nanoparticles and non-radiative coupling from the excited state of the molecule to the localized surface plasmon of the metal nanostructures, which is subsequently radiated by the nanostructures 11,12 . Coumarins are phytochemical and are useful laser dyes due to their high quantum yield of fluorescence and high photostability 14 . The substituents on coumarin affect electronic absorption and emission maxima as well as quantum yields and life time 15 . Upon association/deposition of the fluorophore on the surface of metal nanoparticles, both emissivity and the excited-state lifetime of vicinal fluorophores are affected 16 . Herein we investigate the photophysical properties of pendant coumarinyl motif of embedded CAI-(C n H 2n+1 ) 2 + Br - on GNPs. 1,3-dialkyl-2-(coumarinyl- 6-azo)imidazolium bromide (CAI-(C n H 2n+1 ) 2 + Br - ) have been used for stabilization of GNPs. The properties and size of GNPs are examined by spectroscopic data and TEM images. Materials and Methods Coumarin was used to synthesize 6-aminocoumarin. 2-(coumarinyl-6-azo)imidazole was synthesized by reported procedure 12 . Imidazole and coumarin were of AR grade obtained from SRL, India. H[AuCl 4 ], sodium borohydride and tetraoctylammonium bromide ((C 8 H 17 ) 4 N + Br - , TOAB) were from Sigma-Aldrich. 1-Bromo-n-alkanes (n-C 6 H 13 -1-Br, n-C 9 H 19 -1-Br, n-C 12 H 25 -1-Br, C 15 H 31 -1-Br and C 18 H 37 -1-Br) were purchased from Sigma-Aldrich and used as received. Diethyl ether and 2-propanol (SRL, India) were distilled before use. Microanalytical data (C, H, N) were collected on Perkin–Elmer 2400 CHNS/O elemental analyzer.