Growth of In Situ Functionalized Luminescent Silver Nanoclusters by Direct Reduction and Size Focusing

Abstract

We have used one phase growth reaction to prepare a series of silver nanoparticles (NPs) and luminescent nanoclusters (NCs) using sodium borohydride (NaBH(4)) reduction of silver nitrate in the presence of molecular scale ligands made of polyethylene glycol (PEG) appended with lipoic acid (LA) groups at one end and reactive (-COOH/-NH(2)) or inert (-OCH(3)) functional groups at the other end. The PEG segment in the ligand promotes solubility in a variety of solvents including water, while LAs provide multidentate coordinating groups that promote Ag-ligand complex formation and strong anchoring onto the NP/NC surface. The particle size and properties were primarily controlled by varying the Ag-to-ligand (Ag:L) molar ratios and the molar amount of NaBH(4) used. We found that while higher Ag:L ratios produced NPs, luminescent NCs were formed at lower ratios. We also found that nonluminescent NPs can be converted into luminescent clusters, via a process referred to as "size focusing", in the presence of added excess ligands and reducing agent. The nanoclusters emit in the far red region of the optical spectrum with a quantum yield of ~12%. They can be redispersed in a number of solvents with varying polarity while maintaining their optical and spectroscopic properties. Our synthetic protocol also allowed control over the number and type of reactive functional groups per nanocluster.

The pH-switchable agglomeration and dispersion behavior of fluorescent Ag nanoclusters and its applications in urea and glucose biosensing

“…However, the reported etching methods to obtain Au and Ag NCs always involve toxic organic solvents, rigorous reaction conditions (for example, oxygen-free and high temperature) and complicated procedures. [13][14][15] Among numerous studies on metal nanoclusters, the aggregation behavior of small nanoparticles has attracted widespread attention in recent years, including aggregation-induced optical property changes and their applications, the effects of aggregation on biocompatibility and toxicity, and the shaping of nanocluster supracrystals by controlling the aggregation process. [16][17][18][19] Agglomeration-induced emission (AIE) of fluorescent metal nanoclusters, usually triggered by inorganic ions or special solvents, is becoming an important research focus, [20][21][22] and the reversible AIE of the nanoclusters has drawn the interest of researchers.…”

Efficient photoinduced charge transfer in chemically-linked organic-metal Ag-P3HT nanocomposites

“…Hence, it may be concluded that the PL at 530 nm originates from Ag-P3HT and not from thermal degradation of P3HT or Ag NPs. Some research groups reported silver nanoclusters with diameters smaller than 2 nm are luminescent and their fluorescence maximum resides around 680 nm [16,23,24]. Since it was not observed in the PL spectra, we conclude that the luminescent Ag nanoclusters were not formed.…”

“…Longer conjugation length in the polymer backbone results in a smaller I sym /I asym ratio. While the ratio is only 2 for pristine P3HT [16], it is 10.34 for Ag-P3HT, which suggests the conjugation length is decreased in the chemically-linked Ag-P3HT nanocomposite. The conjugation length is related to the luminescent properties of P3HT derivatives [20].…”

“…Steady-state PL measurements were carried out using a PG2000-Pro-EX spectrometer with an excitation wavelength of 400 nm. Timeresolved fluorescence characterization was performed using the time-correlated singlephoton-counting (TCSPC) technique [16]. The excitation wavelength is 400 nm from the second harmonic output of a Ti: sapphire laser (Maitai HP, Spectra-Physics) at 80 MHz.…”

“…This is similar to the processes in a copolymer [32], except that the separate phase domain of Ag-P3HT is much larger than the one in a copolymer. Interchain interactions associated with shorter decay times relative to the intrachain counterpart are enhanced due to the neighboring P3HT chains that are chemically anchored to a same silver NP [16]. Nevertheless, it needs to be stressed that the above descriptions are just qualitative analysis on the chemically-linked Ag-P3HT structure.…”