Inquiring the mechanism of formation, encapsulation, and stabilization of gold nanoparticles by poly(vinyl pyrrolidone) molecules in 1-butanol
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We present a plausible mechanism of formation, encapsulation, and stabilization of gold nanoparticles (GNPs) in presence of poly(vinyl pyrrolidone) (PVP) in 1-butanol in support of UV–visible, Raman, Fourier transform infrared spectroscopy (FTIR), zetapotential, X-ray photoelectron spectrum (XPS), and transmission electron microscopy. A surface plasmon resonance band at 533 nm in the UV–visible spectrum reveals formation of ~20 nm spherical GNPs in the non-hydrocolloid. In the FTIR spectrum, selective enhancement in the intensity of C–H stretching and red-shift in the C=O band suggests that PVP encapsulate GNP by an interaction between PVP and GNP that occurs via O-atom of pyrrolidone ring. Raman and XPS spectrum well supports the findings of FTIR spectrum. Zeta potential of −15.22 mV at 7.5 pH found in PVP-capped GNP strongly recommends the role of electrosteric effect towards the observed colloidal stability. Microscopic image demonstrates a thin coating of amorphous PVP layer around GNPs in a core–shell structure. Probing the mechanism of formation, encapsulation, and stabilization of GNP could provide essential information for development of bimetallic NPs for catalytic applications.
KeywordsGold nanoparticles Charge transfer Electrosteric effect Zetapotential
Synthesis of therapeutic agent like gold nanoparticles (GNPs) of various architectures continues to be of immense academic and technical interest owing to possession of inimitable opto-electronic, magnetic, physical, chemical, and medicinal properties (Alexandridis 2011; Daniel and Astruc 2004; Hoppe et al. 2006). However, all these properties strongly depend on the type of reducing and/or stabilizing agent used in the reaction medium, synthetic route, and reaction conditions (Alexandridis 2011; Daniel and Astruc 2004). In recent years, researchers around the globe are showing plenty of interest in the use of non-toxic chemicals and environmentally friendly solvents for synthesis of GNPs (Daniel and Astruc 2004; Goy-López et al. 2010; Xian et al. 2012; Zhou et al. 2009). Out of various fundamental strategies, the two methods tested widely so far: (1) bottom-up and (2) top-down approach, the in situ synthesis of NPs by chemical reduction of gold ions in presence of a polymer matrix—a bottom-up technique, is an extensively used synthetic route to obtain stable GNPs in both aqueous and non-aqueous medium (Alexandridis 2011; Goy-López et al. 2010; Xian et al. 2012; Zhou et al. 2009). Soon after the pioneer work of Helcher (1718) on the synthesis of GNPs in presence of a polysaccharide, scientists and academia felt that polymers could be used as efficient stabilizing/reducing agent in the development of inorganic–organic nanostructure materials of various morphologies and sizes of ones interest (Hoppe et al. 2006; Goy-López et al. 2010; Xian et al. 2012; Zhou et al. 2009). In the synthesis of metal NPs, the most commonly used polymers include poly(vinyl alcohol), poly(vinyl pyrrolidone) PVP, and poly (ethylene glycol) (Hoppe et al. 2006; Goy-López et al. 2010; Tripathy et al. 2009; Xian et al. 2012; Zhou et al. 2009). On the other hand, the increased interest on the use of PVP as capping/reducing/nucleating agent has been increased a lot owing to its non-toxicity, good biodegradability, excellent film forming ability and easy processability, superb stabilizing ability, mild reducing ability, and outstanding solubility in various polar solvents (Alexandridis 2011; Hoppe et al. 2006; Xian et al. 2012; Zhou et al. 2009). It was reported that PVP reduce gold ions to Au atom and subsequently encapsulate those metal NPs via O-atom of pyrrolidone ring (Hoppe et al. 2006; Xian et al. 2012; Zhou et al. 2009). PVP forms a charge transfer (CT) complex with Pt NPs via nonbonding electron transfer from the O-atom to the electron-deficient NP (Borodko et al. 2006). As reported by Xian et al., PVP ligand interacts with Pd NPs through O-atom of pyrrolidone ring.
Although a lot of research has been done and still going on the wet chemical synthesis of stable GNPs in presence of homo-polymer/block co-polymer, a small number of articles are available on the mechanistic study of their synthesis, encapsulation, and stabilization (Alexandridis 2011; Hoppe et al. 2006; Goy-López et al. 2010; Sakai and Alexandridis 2004, 2005a; Zhou et al. 2009). Schrinner et al. (2007) investigated the mechanism of formation of highly stable amorphous GNPs of about 1 nm diameter in presence of cationic polyelectrolyte brushes carrying chains of poly(2-aminoethyl methacrylate hydrochloride). They ascribed the excellent stability of GNPs to a strong attraction between negatively charged GNPs and positively charged polyelectrolyte chains. Abyaneh et al. proposed a multi-step mechanism of formation of Au cluster from AuCl4− ions via a free radical in presence of poly(methyl methacrylate) (PMMA) in a polar medium under UV irradiation. Photoreduction of AuCl4− ions offers GNPs obtained via PMMA-assisted growth process. A three-step mechanism of formation of GNPs from AuCl4− was reported in presence of a block co-polymer (Goy-López et al. 2010; Sakai and Alexandridis 2004, 2005a). Block co-polymer encapsulated GNPs formed from AuCl4− ions comprises three major steps: (1) reduction of AuCl4− ions to Au atoms/GNPs by block co-polymer with formation of oxidized organic residues; (2) chemisorption of block co-polymer on the surface of GNP and further reduction of AuCl4− ions on the surface of these gold clusters; and (3) growth of GNPs and finally stabilization by block co-polymer. As different mechanisms have been proposed for the synthesis of GNPs in presence of PVP in both aqueous and non-aqueous medium by various research groups, studying the formation of stable GNPs in presence of mild reducing agent like PVP remains a subject of debate to scientific communities. As proposed by Hoppe et al., synthesis of GNPs in presence of PVP is attributed to the following two reactions: (1) direct abstraction of H-atoms from PVP by the gold ion and (2) reduction of metal precursor by organic macroradicals formed by degradation of PVP. However, Zhou et al. proposed a similar mechanism of formation of GNPs, but in presence of base (i.e., NaOH). Here, the base assists in the degradation of PVP to form highly reactive macroradicals which are responsible for the reduction of Au3+ to Au0. Ram and Fecht had proposed a two-step mechanism of formation of GNPs in the presence of PVP in an aqueous medium. In the first step, the AuCl4− ion gets adsorbed by PVP via O-atom to form an ion–polymer unstable complex. The unstable complex later on breaks to form Au atom and a chlorinated PVP by-product in a local thermally induced transfer reaction in the second step. A mechanism of GNPs formation in presence of PVP in ethylene glycol (EG) under UV irradiation as proposed by Eustis et al. (2005) involves the reduction of the excited Au3+ to Au2+ by EG. This reaction is followed by disproportionation of Au2+ to Au3+ and Au+. Now both reduction of Au+ by EG and disproportionation reaction lead to formation of Au(0). Then, suitable nucleation and growth of Au atom form GNPs.
In this report, we proposed a step-wise path involved in the formation, encapsulation, and stabilization of GNPs in presence of PVP in an organic medium (i.e., 1-butanol). The proposed mechanism is well supported by UV–visible, Raman, Fourier transform infrared (FTIR), X-ray photoelectron (XPS) spectrum, zetapotential measurement, and transmission electron microscope (TEM).
Materials and methods
We used reagent-grade Au(OH)3 powder purchased from Alfa Aesar. It was dissolved in dilute nitric acid to prepare a stock solution of 0.05 M Au(NO3)3. PVP (25 kDa) and 1-butanol were purchased from Sigma Aldrich and Merck, respectively. The chemicals were used as received without further purification.
Preparation of gold colloids
At first, we prepared a mother solution of PVP (4 g/L) in 1-butanol by mechanical stirring for 3 h at 60–70 °C. After this, a specific volume of gold nitrate solution was added drop-wise during stirring to 5 mL of 40 g/L PVP solution taken in a 25-mL beaker maintained at 50 °C. After continuous hot magnetic stirring for 20 min, a purple colored nanocolloid containing a specific concentration of Au NPs with 40 g/L PVP molecules was obtained in 1-butanol. Stable colloids thus obtained were studied using UV–visible, zeta potential, FTIR, XPS, Raman, and TEM.
The UV–visible spectrum of Au non-hydrocolloid with PVP molecules was measured on a Perkin Elmer double beam spectrophotometer (LAMBDA 1050). The sample was filled in a transparent cell of quartz (10 mm path length) and the spectrum was recorded against a reference of 1-butanol with 40 g/L PVP in an identical cell. FTIR spectra have been studied of the solutions with a Thermo Nicolet Corporation FTIR spectrometer (Model NEXUS-870). The spectra have been recorded in an attenuated total reflectance (ATR) mode using a ZnSe crystal as a sample holder. X-ray photoelectron spectrum (XPS) was collected on a VG ESCALAB MK-II spectrometer with a monochromatic Al Kα source (hν = 1486.6 eV) operated at 10 kV and 20 mA at 10−9 Pa. We studied the Raman spectra for the aqueous PVP solution and PVP with GNPs by using a highly sensitive Renishaw inVia Raman microscope in conjunction with a high-sensitivity ultra-low noise CCD detector (RenCam) and an Ar+ ion laser source (λex = 514.5 nm with a maximum 50 W power). Research-grade Leica microscopes are integrated with systems to ensure a high optical efficiency and high stability. Sample for XPS was prepared by drop-casting a drop of Au colloid with PVP in 1-butanol on silicon substrate and dried in desiccators by keeping overnight at room temperature. Microscopic images of PVP-encapsulated GNPs were studied with a high-resolution transmission electron microscope (HRTEM) of JEM-2100 (JEOL, Japan) operated at an accelerating voltage of 100 kV. The samples were prepared by placing one drop of diluted solution on a carbon coated 400-mesh copper grid and then allowing the samples to dry in desiccators overnight at room temperature.
Results and discussion
UV–visible, Raman, and FTIR spectra
Zeta potential and XPS spectra
Mechanism of formation, encapsulation, and stabilization of GNPs
We proposed mechanism of formation, encapsulation, and stabilization of GNPs in presence of PVP molecules in a non-aqueous medium in support of UV–visible, Raman, FTIR, XPS spectrum, zetapotential, and TEM image. Raman, FTIR, and XPS spectrum of PVP-capped GNP suggests that GNPs are covered by a layer of PVP via O-atom of >C=O group of pyrrolidone ring in a CT complex. Zeta potential measurement suggests that electrosteric effect is responsible for the stability of Au colloid in presence of a macroscopic organic stabilizer. TEM image shows that spherical GNPs are well covered by a layer of PVP molecules in the non-hydrocolloid.
This work has been supported by Silicon Institute of Technology, Silicon hills, Bhubaneswar, India.
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