Intense quenching of fluorescence intensity of poly(vinyl pyrrolidone) molecules in presence of gold nanoparticles
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We study the quenching of fluorescence intensity of 40 g/L poly(vinyl pyrrolidone) PVP molecules by varying the content of gold nanoparticles (GNPs) from 1 to 5 μM in 1-butanol. A profound exponential decay of the emission band intensity in the π ← nπ* band of the PVP molecules at ~392 nm upon gradual addition of the GNPs demonstrates an existence of an excited state interaction of NPs with the PVP molecules in a gold colloid in 1-butanol. Such quenching is caused by the non-bonding electron transfer from the O-atom of carbonyl group of the PVP molecules to the surface of the GNP. X-ray photoemission spectroscopy (XPS) study corroborates the spectroscopic results. A linear Stern–Volmer plot with a quenching constant of 2.23 × 106 M−1 reveals dynamic quenching in a non-aqueous NF. A mechanism of fluorescence quenching was proposed in support of XPS and images taken from hybrid nanostructure using transmission electron microscope. Study on quenching of fluorescence intensity of PVP fluorophore in the presence of GNPs is useful for optoelectronic devices and biosensors.
KeywordsNanoparticle Fluorescence quenching Stern–Volmer plot Electron transfer
Hybrid nanostructures consisting of metal nanoparticles, organic dyes, fluorophores, and oligonucleotides are of recent attraction for various applications such as bioassays (You et al. 2007; Mayilo et al. 2009; Pramanik et al. 2008), biosensors (You et al. 2007), biophotonics (Pramanik et al. 2008; Dulkeith et al. 2002), and biomedicals (Kuo et al. 2012; Zhang et al. 2012). However, gold nanoparticles (GNPs) based composite structures have been receiving considerable attention owing to their stability, biocompatibility, dimensional similarities with biomolecules, and non-toxicity (Pramanik et al. 2008; Kuo et al. 2012; Alexandridis 2011). As the fluorescence quenching technique is an effective method for the study of mechanism of molecular interactions such as excited state interaction and ground-state complex formation, energy and charge transfer, it has been widely used to gather information about biochemical systems (Lakowicz 1999). GNP is found to be an efficient quencher for varieties of light emitting substances (e.g., fluorescent polymers, pyrenes, and organic dyes, etc.), due to its nanometric size and large absorption cross section (Mayilo et al. 2009; Gersten 2005; Ghosh et al. 2004; Dulkeith et al. 2005). GNPs with size ranges from 3 to 22 nm quench the fluorescence intensity of protein Cardiac Troponin T with efficiencies as high as 95 % as compared to bigger particles (Mayilo et al. 2009). Dulkeith et al. (2002) studied the fluroscence quenching of lissamine dye molecules attached to differently sized GNPs using time-resolved fluorescence experiments. A profound quenching by 1 nm GNP was ascribed not only by an increased nonradiative rate but also due to a drastic decrease in the dye’s radiative rate. An experimental verification of the size effect of GNPs on quenching of 1-methylaminopyrene (MAP) was done by Ghosh et al. (2004). As suggested by the authors, the large surface to bulk atom ratio of GNPs is responsible for the efficient quenching of emission intensity of MAP molecules.
In this article, we studied the decrease in the fluorescence intensity of poly(vinyl pyrrolidone) PVP molecules by varying the GNPs content in 1-butanol. The formation of GNPs in a non-hydrocolloid was confirmed by UV–visible, X-ray photoemission spectroscopy (XPS), and transmission electron microscope (TEM). An organic solvent was chosen in this work because such medium founds to provide low interfacial energies needed for a high degree of control during solution and surface processing is required for synthesis of nano colloids (Balasubramaniam et al. 2002). Also it was reported that Au-NPs synthesized in an organic medium offer excellent control over size and possess superior processability (Sugunan et al. 2005).
Materials and methods
Chemicals and instruments
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.
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. X-ray photoelectron spectrum (XPS) was collected on a VG ESCALAB MK-II spectrometer with a monochromatic Al Kα source (hν = 1,486.6 eV) operated at 10 kV and 20 mA at 10−9 Pa. The sample for XPS was prepared by drop-casting a small aliquot of Au colloid with PVP in 1-butanol on silicon substrate and dried in desiccators by keeping overnight at room temperature. The photoluminescence spectra have been recorded with a computer controlled Perkin–Elmer (Model-LS 55) luminescence spectrometer in conjugation with a red sensitive photo multiplier tube detector (RS928) and a high energy pulsed xenon discharge lamp as an excitation source (average power 7.3 W at 50 Hz). A quartz cell of 10 mm width was used as sample holder.
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 dropping 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.
Preparation of PVP-capped GNPs
At first, we prepared an organic mother solution of PVP (40 g/L) in 1-butanol by mechanical stirring for 3 h at 60–70 °C. After cooling the PVP solution to room temperature, a specific volume of gold nitrate solution was added drop wise while stirring to four different batches, each of 5 mL of 40 g/L PVP solution in 1-butanol at 50 °C. After continuously hot magnetic stirring for 20 min, we obtained series of purple colored Au colloids (with intensity increases with GNPs content) consisting of 1, 2, 3, and 5 μM GNPs with 40 g/L PVP molecules, in 1-butanol.
Results and discussion
UV–visible and XPS spectrum
Photoluminescence (PL) spectra
Emission and excitation spectra
GNPs efficiently quench the PL intensity of PVP molecules in 1-butanol. An exponential decay of PL intensity in the π ← nπ* band of PVP molecules at ~392 nm in the presence of GNPs suggest an excited state interaction between GNP and PVP molecules. A large KSV = 2.23 × 106 M−1 reveals a dynamic quenching mechanism. Profound PL quenching of biopolymer in the presence of biocompatible quencher such as GNP is suitable for biosensors, bioassays, biomedical, and biophotonics applications.
This work has been supported by Silicon Institute of Technology, Silicon Hills, Bhubaneswar, India.
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