Fluorescence From Metallic Silver and Iron Nanoparticles Prepared by Exploding Wire Technique
- First Online:
- Cite this article as:
- Alqudami, A. & Annapoorni, S. Plasmonics (2007) 2: 5. doi:10.1007/s11468-006-9019-2
- 395 Views
The observation of intense visible fluorescence from silver and iron nanoparticles in different solution phases and surface capping is reported here. Metallic silver and iron nanoparticles were obtained by exploding pure silver and iron wires in double distilled water. The adsorption of bovine serum albumin protein on the silver nanoparticles showed enhanced fluorescence. The presence of polyvinyl pyrrolidone polymer in the exploding medium resulted in a stabilized growth of iron nanoparticles with enhanced fluorescence intensity. The fluorescence was found to be surface/interface-dependant and is attributed to electronic transitions among characteristic interface energy bands. The magnetic nature of iron nanoparticles was confirmed from the hysteresis measurements.
Key wordsMetal nanoparticlesFluorescenceExploding wire
Metal nanoparticles display novel physical and chemical properties due to surface effect, where most of the particle atoms are just surface atoms . These novel properties have put metal nanoparticles to play an interesting role in materials technology, biomedicines, catalysis, etc. The optical properties of metal nanoparticles are highly influenced by the preparation methods and conditions, which result in particles of various size, shape, and surface stabilization [2, 3]. Moreover, the surface/interface interactions have their signatures in the properties investigated [4, 5]. Among all the known chemical and physical preparation methods, the exploding wire technique is one of the newest and simplest methods for producing metal nanoparticles [6, 7]. The explosion is achieved when a very high current density is applied to a thin metal wire, causing the wire to explode into very small fragments. This process involves wire heating and melting followed by wire evaporation, formation of a high-density core surrounded by low-density ionized corona, coronal compression, and fast expansion of the explosion products . The exploding wire experiments were mostly employed for the generation and investigation of plasma and shock waves. In previous reports, we studied the optical properties of silver nanoparticles prepared by exploding wire technique. These silver nanoparticles showed surface plasmon absorption and plasmonic excited fluorescence [9, 10]. The fluorescence of metal clusters and thin films is well established based on Mooradian’s observation of photoluminescence from bulk copper and gold , and on photoinduced luminescence from the noble metals observed by Boyd . For example, the photoinduced fluorescence from Ag3 clusters during the agglomeration of silver atoms was observed to center around 500 nm, as reported by Ievlev et al. . Dendrimer-encapsulated silver nanodots were also observed to be fluorescent both in aqueous solutions and in films . Most of the observed fluorescence was attributed to radiative recombination of an electron-hole pair between d band and sp-conduction band above the Fermi level. Due to plasmon resonance excitation, the local field created around the nanoparticles is found to modify the observed fluorescence .
Recently , the thermal growth of silver nanoparticles on soda glass was correlated with drastic changes in their photoluminescence intensity. Another model has been proposed to explain the fluorescence from silver colloidal particles, as their surface was chemically changed by deposition of silver ions .
Silver nanoparticles in water were found to have fluorescence peak at 465 nm. The interface electrons were proposed to be the source of the observed fluorescence . Far from the fluorescence from noble metals, Fei et al.  observed for the first time the fluorescence from α-Fe2O3 nanoparticles—coated with a layer of organic molecules. They attributed the fluorescence to bound-exciton emission, as also observed for other typical semiconductor nanoparticles. On the other hand, mesoscopic systems that consist of a spherical metallic core surrounded by an insulating layer were investigated theoretically, where the electromagnetic decay of externally located electrons to the lower-lying unoccupied electronic states located within the metal exhibit a resonance behavior. It was shown that the probability of lowest external electrons to be found in the inner states located within the metal is increasing with the metallic core radius .
In the present study, we report the observation of strong visible fluorescence from silver nanoparticles in different media under plasmonic excitations, and for the first time, we have observed an intense fluorescence from iron nanoparticles under an ultraviolet excitation. Both systems were prepared using the exploding wire technique in pure water and under the same conditions. To study the surface effects on the observed fluorescence, bovine serum albumin (BSA) proteins were introduced to prepare BSA-adsorbed silver nanoparticles, whereas iron nanoparticles were coated with polyvinyl pyrrolidone (PVP) polymer.
Silver and iron wires (99.998%) with diameters of 0.2 mm were exploded in double distilled water using silver and iron plates, respectively, to produce silver and iron nanoparticles. Next, 25 μM PVP polymer (Mn = 40,000, Aldrich) was dissolved in double distilled water, and this was used as the explosion medium for iron wires to produce PVP-coated iron nanoparticles. These wires were exploded by bringing the wire into sudden contact with the plate (purity 99.998%) when subjected to a potential difference of 36 V DC. High current density was allowed to pass through these thin wires, where tensile fracture forces proportional to the square of the current caused the wires to rupture before becoming unduly softened by the ohmic heating. The total mass being exploded was about 0.3 g in each experiment. The explosions in all the experiments were carried out under the same conditions. BSA proteins (SRL, India) were added to the water-based silver nanoparticles solution with total concentration of 10.0 μg/ml under overnight vigorous stirring at 4°C. Part of the silver and iron nanoparticle solutions were centrifuged at 12,000 rpm, and the separated particles were washed and redispersed in cyclohexane.
Powder nanoparticles obtained after filtering were used for the x-ray diffraction studies that were performed with a Philips Analytical X-Ray Diffractometer type PW3710 using Cu-Kα radiation (wavelength 1.54056 Å). Drops from the decanted solutions were allowed to dry on carbon-coated copper grids for electron microscopy imaging using JEOL JEM 2000EX transmission electron microscopy (TEM). Particle size distributions were performed using Photocor-F Dynamic light scattering, Photocor instruments. A UV-2510PC spectrophotometer was used to record UV-visible absorption spectra. The fluorescence emission and excitation spectra were recorded using Edinburgh Analytical Time Resolved Fluoremeter. The magnetic moment (M) of the iron wire and the water-based iron nanoparticles was measured using Lake Shore model 7304 Vibrating Sample Magnetometer.
Transmission electron microscopy
Dynamic light scattering
The UV-visible spectra of iron nanoparticles are shown in Figure 4b. Iron nanoparticles in water showed two absorption peaks at wavelengths of 216 and 268 nm. These two structures are not clear in the case of PVP-stabilized iron nanoparticles. However, strong absorption is observed below 250 nm, whereas the structure at 268 nm in the last case disappeared, but with the appearance of a small hump at about 360 nm. This can be assigned to small-size clusters of PVP-stabilized iron nanoparticles, which is in agreement with reported absorption spectra [22, 23].
Iron nanoparticles in cyclohexane show four absorptions at 207, 224, 252, and 280 nm. It seems that the mean absorption peaks at 216 and 268 nm of the iron nanoparticles in water have been split to four absorptions. The iron nanoparticles in cyclohexane have some selected favorite transitions, instead of broad absorption peaks in the case of water interface. The extinction coefficient of spherical iron nanoparticles having a mean diameter of 10 nm, dispersed in water, has been calculated according to Mie’s theory. The damping term as a function of the particle size was included in the calculations of the dielectric functions of iron nanoparticles . The dielectric functions of bulk iron were taken from Johnson and Christy . The correction was made by inserting a constant that includes the details of the scattering processes and the particle size that give rise to the size dependence. This theoretical curve is in good agreement with those obtained experimentally and in more agreement with the PVP-stabilized iron nanoparticles wherein the average particle size is about 10 nm. The theoretical simulation obtained in the present work is also in agreement with the reported simulations .
Fluorescent iron nanoparticles produced using the exploding wire technique showed a superparamagnetic behavior . Detailed study on the magnetic properties of these iron nanoparticles is presented elsewhere . The saturation magnetization (Ms) values obtained for the water-based iron nanoparticles in the presence and absence of PVP were 25 and 55 emu/g, respectively. Assuming the lognormal size distributions of the water-based iron nanoparticles, the size of magnetic core is calculated according to the method described by Chantrell et al. , and the resultant magnetic core diameter was 4.2 nm for the PVP-stabilized iron nanoparticles and 4.6 nm for the nonstabilized particles. The reduced (Ms) values with respect to the Ms value of the bulk iron wire used for the explosion (which was 222 emu/g) are attributed to the increase in the anisotropy due to size reduction and nanoparticle surface coating.
The exploding wire results in small clusters and evaporated atoms and ions, which are in fact the source of plasma generated during the explosion process. The resulting fragments will be too dense and give rise to high filling factor. Coagulation aggregates are easily formed due to Van der Waals forces or bonding between two clusters approaching each other. The presence of some amounts of stabilizers such as PVP polymer in the explosion medium can terminate the aggregation. Water as a medium will provide fast cooling for the generated particles with energetically favored shapes. Moreover, the high surface area per unit mass of the metal nanoparticle enhances the surface activity and tends to react with hydroxide molecules. Metal nanoparticle surface, in the absence of polymers, thus may get oxidized. This process results in forming some sort of oxide layer. The coating process occurs during and after the particle growth and agglomerations. The oxidation of iron nanoparticles is reflected through the increasing of measured pH value from 6.4 for the distilled water used to 7.14 for the water-based iron nanoparticles solution. The PVP-coated iron nanoparticles solution has a pH value of 6.82. Thus, PVP avoids the oxidation of the nanoparticles.
Iron nanoparticles stabilized by PVP polymer showed an enhanced fluorescence emission as expected because the electron density at the particle surface is enhanced. Unlike water, cyclohexane as a medium will not affect the surface free electrons at all, and hence, the fluorescence intensity is highly enhanced from both silver and iron nanoparticles. However, the selection rule of the electronic transitions is changing. This can be observed through the shift of the fluorescence peaks depending on the medium (environment). A similar explanation was also reported in systems such as silver nanoparticles in water phase  and coated α-Fe2O3 nanoparticles .
Regardless of the electrons at the interface, the emissive center can be the metallic nanoparticle core, as it was concluded that external electrons can decay electromagnetically to internal electronic states located within the metallic core . This process, of course, depends on the metallic core size. The emissive centers can be something else as well. In the case of silver and iron nanoparticles prepared by the exploding wire technique, the emission of plasma light during the explosion process is a clear indication to the generation of ions. Some of these ions may get adsorbed to the surface of the formed nanoparticles and they became the emissive centers. A similar proposal was presented to explain fluorescence emission from silver nanoparticles in a solution exposed to silver ions . The interaction between the emissive centers and the interface environment also has its signature on the observed fluorescence.
Indeed, the explosions were carried out in water and the fluorescence was recorded in air. This may give rise to partial surface oxidation. One suggestion is that partial oxidation of metal nanoparticles leads to the formation of metal oxide clusters on the metal nanoparticle surface. These clusters at the surface are photoactivated by light excitations and might give rise to the observed fluorescence. Photoactivated fluorescence has been previously observed from small silver clusters .
Although the observation of fluorescence from noble metal nanoparticles and nanoclusters is not new, models put to explain the behavior are still not clear enough and, most of the time, are not identical. Furthermore, studying the fluorescence of metallic nanoparticles should be extended to other applicable systems. Here, the fluorescence of iron nanoparticles with their magnetic nature is observed for the first time. We report the fluorescence from silver nanoparticles prepared by the same technique using the same mediums. The fluorescence of silver nanoparticles more or less agrees with the literature. In fact, the fluorescence from iron nanoparticles under investigation is much more than that from silver nanoparticles.
The possible explanation of these emissions is the creation of surface/interface energy bands where the probability of some allowed transitions is increased. Fluorescent magnetic nanoparticles are a totally new discovery. The metal nanoparticles obtained may find applications in the field of water purifications and biomedicines.
We would like to acknowledge Dr. N. C. Mehra and Mr. Raman of the University Science Instrumentation Center and Mr. P. C. Padmakshan of the Geology Department, University of Delhi, for recording TEM, UV-visible, and x-ray diffraction data, respectively, and Prof. K. Muralidhar and Mr. Vineet Sharma, of the Department of Zoology, University of Delhi, for their help in carrying out the BSA adsorption experiments and their helpful discussions. We also thank Dr. N. K. Chaudhary of the Institute for Nuclear Medicines and Allied Sciences, Delhi, for recording the fluorescence spectra and Dr. R. K. Kotnala of the National Physical Laboratory, Delhi, for carrying out the magnetization measurements. We would also like to acknowledge the Department of Science and Technology, India, for the funding throughout the project (SR/S5/NM-52/2002) from the Nanoscience and Technology Initiative program.