Synthesis and stability of fluorescent gold nanoparticles by sodium borohydride in the presence of mono-6-deoxy-6-pyridinium-β-cyclodextrin chloride

Gold nanoparticles in the range of 5-6 nm were synthesized by the reduction of gold(III) chloride trihydrate by sodium borohydride (SBH) in the presence of newly synthesized mono-6-deoxy-6-pyridinium-β-cyclodextrin chloride (p-βCD). NMR, mass spectroscopy, and UV-vis spectroscopy illustrated that SBH would simultaneously reduce both p-βCD and gold salt even though the reduction of the latter occurs more rapidly. Resulting gold nanoparticles were capable of oxidizing the reduced p-βCD, leading to the formation of the p-βCD-gold complex via hydrogen bonding and ionic interaction. Gold nanoparticles were synthesized from a much higher concentration (1.0 mM vs 0.25 mM in the absence of p-βCD) of gold salt and were not susceptible to aggregation by NaCl, phosphate (pH 4-10), acetate, citrate, and borate. The p-βCD-gold nanoparticle assembly displayed intensified fluorescence with emission at 498 nm when excited at 470 nm, a phenomenon known as metal-enhanced fluorescence. The gold nanoparticles acted as electron acceptors and controlled the pathways of the excited-state deactivation. Surface binding of the pyridinium moiety of p-ßCD to gold nanoparticles suppressed the intramolecular photoinduced electron transfer from the lone pair of the nitrogen atom to the aromatic ring and thereby increased the efficiency of radiative deactivation, leading to a fluorescence enhancement. The p-βCD-gold nanoparticle system could be exploited for various fluorescence chemosensing and biosensing schemes, especially for applications demanding long observation times without photobleaching.