Facile synthesis of SERS active Ag nanoparticles in the presence of tri-n-octylphosphine sulfide
Research highlights
▶ Hydrophobic Ag nanoparticles were synthesized by using AgNO3, tri-n-octylphosphine and sulfur powder in process. ▶ Tri-n-octylphosphine was used as solvent, reducing agent and stabilizer in the synthetic process. S could chelate with excessive tri-n-octylphosphine to form trioctylphosphine sulfide, which could adjust Ag nanoparticles’ growth due to its strong capping ability and served as second capping agent. ▶ Through surface exchange with 3-mercaptopropanoic acid, the hydrophobic Ag nanoparticles could be transformed into hydrophilic ones. ▶ The synthetic strategy was simple and the as-synthesized Ag nanoparticles could be used as substrates for SERS detection.
Introduction
Noble metal nanoparticles (NPs), especially Ag and Au ones, have been a subject of intensive research in the past decades. These NPs show a strong enhancement of absorption and scattering of visible or infrared light in resonance with their surface plasmon resonance (SPR) frequency, which lead to many promising applications, such as surface-enhanced Raman scattering (SERS) [1], bioimaging [2], photothermal therapy [3], optoelectronics, etc. [4]. Unique physical and chemical properties of noble metal NPs are highly dependent on their size, shape, and environment of the particles [5]. As a result, great attention has been directed toward the control of the noble metal NPs’ size, size distribution and their morphology. For Ag NPs, various synthetic strategies have been developed in order to get desired properties and functions. Ordinarily, Ag nanostructures are obtained by reducing AgNO3 with sodium citrate, sodium borohydride or by the liquid–liquid two-phase method [6], [7], [8], [9]. Recently, the synthesis of Ag NPs also performed in organic solvents using strong or mild reducing agents in order to get desired stability and dispersion. For example, Itoh et al. utilized an oxalate-bridged silver–oleylamine complex with CO2 evolution to produce Ag NPs with ∼11 nm dimension [10]. Nath et al. synthesized hexadecylamine (HDA)-capped Ag organosols which were stable for over a year [11]. Chen and Gao reported the preparation of nearly monodispersed Ag NPs using tri-n-octylphosphine (TOP) as the surfactant and stabilizer beyond 180 °C [12]. All these results show that significant efforts have been focused on designing new synthetic protocols which demand reducing agents effectively reduce Ag salts and provide a robust coating on the Ag nanostructures.
Very recently, a simple strategy for the synthesis of various semiconductor nanocrystals by inducing metal salts and sulfur (S) or selenium powder in TOP was developed in our group [13], [14], [15]. When silver nitrate and a small amount of S powder were added into above system, Ag NPs would be obtained below the hydrolysis temperature of trioctylphosphine sulfide (TOPS). Herein, we described the preparation of hydrophobic Ag NPs by adjusting Ag:S molar ratios. S could react with TOP to form TOPS, which limited Ag NPs’ growth due to its strong capping ability. Besides, through surface exchanging with 3-mercaptopropanoic acid (3-MPA), the hydrophobic Ag NPs could be converted to hydrophilic ones. Both the hydrophobic and the hydrophilic Ag NPs modified glass surfaces could be served as active substrates for SERS applications. The SERS spectra of 4-aminothiophenol (4-ATP) molecules were obtained on theses surfaces. This work offered a general approach for the synthesis of Ag NPs, which might be important for optical detection application.
Section snippets
Materials
TOP, 3-MPA and 4-ATP were purchased from Alfa Aesar Chemicals. AgNO3, S powder and all solvents were purchased from Beijing Chemical Reagents Co. All reagents were of analytical grade and used without further purification.
Synthesis of hydrophobic Ag NPs
The Ag:S molar ratio varied from 1:0 to 1:50 by keeping Ag amount unchanged. An appropriate amount of S powder, AgNO3 (0.15 mmol) and TOP (4 mL) were added into a 25 mL three-neck flask. Under nitrogen protection, the mixture was heated to 150 °C under magnetic stirring. The
Results and discussion
Fig. 1 presents the TEM images, size distribution histograms of hydrophobic and hydrophilic Ag NPs and their corresponding XRD patterns. The formation of Ag NPs was deeply influenced by experimental parameters. When no S powder was used in the synthetic process, the product's size was large and only irregular solid Ag nanostructures were obtained (Fig. 1A). It seemed that TOP was a relative weak capping reagent. When Ag:S molar ratio was 1:10, the size of the as-synthesized Ag nanostructures
Conclusion
A simple and effective approach for the synthesis of Ag NPs was reported. TOP served as reducing agent and weak capping agent in the synthetic process. The amount of S could affect the morphology of Ag NPs because of S could interact with TOP to form TOPS which had higher capping ability. When the molar ratio of Ag:S was set at 1:40, relative uniform and dispersed hydrophobic Ag NPs could be synthesized effectively. The hydrophobic Ag NPs could be transformed into hydrophilic ones through
Acknowledgements
This work was supported by the National Nature Science Foundation of China (No. 20975012) and the 111 Project in China (B07012).
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