J. Phys. Chem. C 2008, 112, 443-451
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Synthesis and Stability of Fluorescent Gold Nanoparticles by Sodium Borohydride in the Presence of Mono-6-deoxy-6-pyridinium-β-cyclodextrin Chloride Keith B. Male,† Jianjun Li,‡ Ching Chi Bun,§ Siu-Choon Ng,§ and John H. T. Luong*,† Biotechnology Research Institute, National Research Council Canada, Montreal, Quebec, Canada H4P2R2, Institute of Biological Sciences, National Research Council Canada, Ottawa, Ontario K1A 0R6, and School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, Singapore 639798, Singapore ReceiVed: October 12, 2007
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-βCD 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.
Introduction Nanomaterials with size-dependent physical properties have been developed for diversified and novel applications. Gold nanoparticles, in particular, are very attractive for research in nanotechnology because of their appealing features. Nontoxic gold colloids, readily and inexpensively prepared by chemical reduction of HAuCl4, are capable of forming active complexes with many biological substances.1 Depending on the reducing agent used, the colloid particle size can vary from 1 to 100 nm.2 The sodium citrate reduction procedure pioneered by Frens3 is one of the most commonly cited methods for colloidal gold synthesis to produce nanoparticles with sizes of 12-64 nm, depending upon the citrate/HAuCl4 ratio. In immunocytochemistry, the use of smaller gold nanoparticles is preferred because steric hindrances are often encountered with larger particles, which can significantly reduce the number of sites for labeling.1 Smaller gold nanoparticles (4-6 nm) can be constructed by the reduction of gold(III) chloride trihydrate with sodium citrate in the presence of RCD, βCD, and γCD.4 Alternatively, smaller particles (3-6 nm) are often made using other reducing agents such as white phosphorus, sodium borohydride (SBH), or a mixture of trisodium citrate and tannic acid.2,5 The reduction by SBH1 is simple, rapid, and can be performed at room temperature, unlike the citrate reduction procedure,3,6 which requires refluxing at boiling temperatures and ultra-clean * To whom correspondence should be addressed. † Biotechnology Research Institute, National Research Council Canada. ‡ Institute of Biological Sciences, National Research Council Canada. § Nanyang Technological University.
glassware. However, the maximum gold salt concentration using the SBH reduction procedure must be kept below 0.25 mM, otherwise aggregation occurs quickly, compared to 1 mM for the citrate reduction. Although many gold colloid preparations are very stable upon long-term storage, aggregation/precipitation difficulties are encountered when they are added to various salts and buffer systems. In some cases this behavior can be alleviated by the addition of specific stabilizers. The optical and electrical properties of colloidal gold nanoparticles are known to be dramatically affected by their size, shape, and surrounding surface environments.7-9 The effect of rodlike gold metal structures on the optical properties has been extensively studied.10-13 Fluorescence of gold nanorods (aspect ratios of 2.0-5.4, width of 20 nm) has been reported with an excitation wavelength of 480 nm and emission maximum ranging from 548 to 588 nm, respectively.14 However, the quantum efficiency of such nanorods is very low, limiting the widespread use of their fluorescence properties in biosensing. Although gold nanoparticles have not been reported to exhibit fluorescence under these conditions, photoluminescence at 440 nm has been observed for small gold nanoparticles (5 nm) when excited at a wavelength of 230 nm.15 New ammonium-functionalized cyclodextrins have been prepared by the displacement of 6-tosyl-β-cyclodextrin with alkylimidazoles, pyridine, and alkylamines in dimethyl formamide.16 Similar to ionic liquids, these cationic cyclodextrins proved to be highly successful stationary phases for the enantioseparation of anionic analytes due to strong electrostatic interactions. Imidazolium ionic liquids have been reported to exhibit fluorescence.17 Recently alcohol ionic liquids have been
10.1021/jp7099515 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/22/2007
444 J. Phys. Chem. C, Vol. 112, No. 2, 2008 introduced during the preparation of gold nanoparticles (4.3 nm) and act as both reductants and protective agents.18 This study describes the preparation and characterization of concentrated, stable, and fluorescent gold nanoparticles by reduction of gold(III) chloride trihydrate with sodium borohydride in the presence of two cationic β-cyclodextrins: mono-6-deoxy-6-(3-methylimidazolium)-β-cyclodextrin chloride (m-βCD) and mono-6deoxy-6-pyridinium- β-cyclodextrin chloride (p-βCD). The gold nanoparticle size was confirmed by both atomic force microscopy (AFM) and transmission electron microscopy (TEM), while the fate of the CD during the nanoparticle preparation was elucidated by mass spectrometry (MS) and NMR. Results and Discussion Mass, UV-Vis, and Fluorescent Spectroscopy. The extract mass spectrum of p-βCD is presented in Figure S1a (Supporting Information), in which a predominant ion was detected at m/z 1197, consistent with the calculated molecular weight of p-βCD.16 MS/MS of the precursor ion at m/z 1197 shows a series of ions at m/z 1034.3, 872.3, 710.3, 548.3, 386.2, and 224.2, corresponding to the consecutively loss of a hexose residue (Figure S1b). To confirm the structure of ion m/z 224.2 as the pyridinium-hexose, an MS/MS/MS experiment was performed. The peak at m/z 80 confirms the presence of the pyridinium ring in p-βCD (Figure S1c). Gold nanoparticles formed by the reduction of gold(III) chloride trihydrate (0.25 mM) by sodium borohydride (1.25 mM) in the presence of p-βCD (0.50 mM) exhibited a red/burgundy color with initial absorption at 520525 nm (Figure S2A, curve a, Supporting Information). This feature is commonly known as the surface plasmon resonance (SPR) peak and dependent on the particle size. Within the first hour, a new absorption peak was observed at 326 nm, and this peak intensified with time until a maximum was achieved after 1-2 days (curves b-e of Figure S2A). Concurrently, the absorption peak at 270 nm due to p-βCD (Figure S2B, curve a) decreased noticeably. In addition the absorption peak at 525 diminished and shifted slightly to a shorter wavelength of 516520 nm with the solution taking on a more reddish color. The peak at 326 nm does not arise from either of the starting materials as indicated in Figure S2B (curves a and b). Gold nanoparticles formed in the presence of βCD (0.5 mM, curve c) or in the absence of cyclodextrins (curve d) also did not exhibit the peak at 326 nm observed with p-βCD (curve e). As discussed later, this peak could be assigned to the slow formation of stable p-βCD-gold nanoparticle complexes via hydrogen bonding and ionic interaction. The pyridine ring was expected to be included in the hydrophobic cavity of p-βCD to form a stable complex with gold nanoparticles as well as enhance the energy transfer from the lone pair electron of the nitrogen atom in the pyridine ring to gold nanoparticles. In contrast, gold nanoparticles formed in the absence of cyclodextrin initially absorb at 500 nm (orange color), and then the absorption peak shifts to longer wavelengths and stabilizes at 515-517 nm (orange/red color) as the particles form larger colloids. After 24 h, the reaction is complete, and there is a marked reduction in the particle heterogeneity.2 As reported by Liu et al.,4 the addition of high concentrations of βCD (10 mM) shifts the final peak to a shorter wavelength (510-512 nm), and the size of the gold nanoparticles was reduced from 6.0 to 2.5 nm. Gold nanoparticles formed by the reduction of gold(III) chloride trihydrate (0.25 mM) by sodium borohydride (1.25 mM) in the presence of m-βCD (0.50 mM) exhibited a burgundy/purple color with initial absorption at 536 nm (figure not shown). Unlike the synthesis with p-βCD, this absorption
Male et al. peak at 536 diminished and shifted to a longer wavelength (546 nm) with the solution taking on a more purple color with time. The gold nanoparticle solution with m-βCD tended to sediment with time, although it could easily be resolubilized by gentle shaking. The gold nanoparticles synthesized in the presence of p-βCD developed fluorescence with time, corresponding to the emergence of the absorption peak at 326 nm. The maximum emission was detected at 498 nm when the excitation was set at 470 nm (PMT-550 V), and the signal (sample diluted 10-fold) reached a plateau after 2-3 days. The excitation spectrum (emission 498 nm) was broad, and the sample when excited at 425 nm still had an emission signal 25% of the maximum. The optimum excitation/emission wavelengths are very similar to those reported for gold nanorods.14 The starting materials (p-βCD, SBH, and HAuCl4) do not fluoresce under these conditions. However, 0.5 mM p-βCD exhibited a very weak fluorescence emission at 428 nm when excited at 375 nm using a PMT of 800 V and pyridine exhibits similar fluorescence under these conditions. Similar excitation/emission wavelengths have been reported for imidazolium ionic liquids.17 No fluorescence was observed with the gold nanoparticles synthesized in the presence of m-βCD. To exploit the fluorescence property of gold colloids, all subsequent experiments focused on the use of p-βCD. Metal-enhanced fluorescence of p-βCD is of particular interest since this phenomenon has been known to occur when a fluorophore is positioned in close proximity (
