Gold Nanoparticles – Surface Plasmon Resonance - Revision history

Cmditradmin at 22:03, 7 July 2010

2010-07-07T22:03:14Z

← Older revision		Revision as of 15:03, 7 July 2010
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	Not to downgrade numerous research activities throughout the 20<sup>th</sup> century, but one late report appears to be an important milestone in the gold nanoparticle research. Published by Brust et al. in 1994, it described a successful preparation of gold nanoparticles in organic solvents with the use of organic thiols as stabilizers of the resulting colloidal gold.<sup>16</sup> This started a renaissance in gold-nanoparticle research as the organic-soluble particles could be treated like typical organic compounds; i.e., standard organic chemistry techniques could be used for the manipulation of these species. Additionally, the synthetic accessibility and flexibility of the organic-thiol stabilizers made it possible to systematically study a variety of effects of the stabilizer molecules on the physical properties of the gold particles and vice versa.'''		Not to downgrade numerous research activities throughout the 20<sup>th</sup> century, but one late report appears to be an important milestone in the gold nanoparticle research. Published by Brust et al. in 1994, it described a successful preparation of gold nanoparticles in organic solvents with the use of organic thiols as stabilizers of the resulting colloidal gold.<sup>16</sup> This started a renaissance in gold-nanoparticle research as the organic-soluble particles could be treated like typical organic compounds; i.e., standard organic chemistry techniques could be used for the manipulation of these species. Additionally, the synthetic accessibility and flexibility of the organic-thiol stabilizers made it possible to systematically study a variety of effects of the stabilizer molecules on the physical properties of the gold particles and vice versa.'''


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	===Gold Nanoparticles – Surface Plasmon Resonance===		===Gold Nanoparticles – Surface Plasmon Resonance===
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	As described above, the rather complicated cascade of processes leading to the excited-state deactivation of gold nanoparticles is mostly non-radiative, implying fast and efficient heating of the close surroundings of the nanoparticle right after photoexcitation. This heating can be so efficient that it can actually cause irreversible changes in the nanoparticle morphology due to melting. This behavior has been exploited for a variety of potential applications including information storage,<sup>29</sup> and cancer diagnostics and therapy.<sup>30</sup>		As described above, the rather complicated cascade of processes leading to the excited-state deactivation of gold nanoparticles is mostly non-radiative, implying fast and efficient heating of the close surroundings of the nanoparticle right after photoexcitation. This heating can be so efficient that it can actually cause irreversible changes in the nanoparticle morphology due to melting. This behavior has been exploited for a variety of potential applications including information storage,<sup>29</sup> and cancer diagnostics and therapy.<sup>30</sup>


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	===Noble Metal Nanoparticles – Local Field Effects===		===Noble Metal Nanoparticles – Local Field Effects===
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	The potentially important consequences of the enhancements in nonlinear spectroscopic response of molecules due to the proximity of metallic nanostructures form one of the points of motivation for the work on gold nanoparticles presented in this thesis.		The potentially important consequences of the enhancements in nonlinear spectroscopic response of molecules due to the proximity of metallic nanostructures form one of the points of motivation for the work on gold nanoparticles presented in this thesis.


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	===Gold Nanoparticle / Organic Dye Systems and their Photophysics===		===Gold Nanoparticle / Organic Dye Systems and their Photophysics===

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	===Energy Transfer to the Nanoparticle and Radiative Rate Modification===		===Energy Transfer to the Nanoparticle and Radiative Rate Modification===

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	It should be noted that the G-N theory fails to describe systems in which the distance between the luminescent dye and the nanoparticle surface is small (i.e. on the order of a nanometer). For example the experimental data of Dulkeith et al., which was previously mentioned, showed much smaller nonradiative rates than those calculated from G-N theory.<sup>44,53</sup> Also, radiative rates calculated for a cyanine molecule (Cy5) at different distances from a gold nanoparticle surface were overestimated when compared to the measured values.<sup>53</sup>		It should be noted that the G-N theory fails to describe systems in which the distance between the luminescent dye and the nanoparticle surface is small (i.e. on the order of a nanometer). For example the experimental data of Dulkeith et al., which was previously mentioned, showed much smaller nonradiative rates than those calculated from G-N theory.<sup>44,53</sup> Also, radiative rates calculated for a cyanine molecule (Cy5) at different distances from a gold nanoparticle surface were overestimated when compared to the measured values.<sup>53</sup>


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	====Electron Transfer to the Nanoparticle====		====Electron Transfer to the Nanoparticle====

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	van Herrikhuyzen et al. used a femtosecond transient absorption setup to study the effect of gold nanoparticles (ca. 1.6 nm and 4.1 nm in diameter) on the photophysics of oligo(phenylene-vinylene) (OPV) covalently attached to the nanoparticle surface.<sup>62</sup> Even though the signal from the metallic core of the particles dominated the measured transient absorption traces the authors, were able to conclude that the measured traces were consistent with ultrafast energy transfer, which limited the lifetime of the organic dye to 2 ps. The authors demonstrated no measurable electron transfer taking place in the studied system, thus limiting the number of processes of the excited-state deactivation of the organic dye to the energy transfer.<sup>62</sup>		van Herrikhuyzen et al. used a femtosecond transient absorption setup to study the effect of gold nanoparticles (ca. 1.6 nm and 4.1 nm in diameter) on the photophysics of oligo(phenylene-vinylene) (OPV) covalently attached to the nanoparticle surface.<sup>62</sup> Even though the signal from the metallic core of the particles dominated the measured transient absorption traces the authors, were able to conclude that the measured traces were consistent with ultrafast energy transfer, which limited the lifetime of the organic dye to 2 ps. The authors demonstrated no measurable electron transfer taking place in the studied system, thus limiting the number of processes of the excited-state deactivation of the organic dye to the energy transfer.<sup>62</sup>


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	====Intermolecular Interactions====		====Intermolecular Interactions====
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	# '''''References'''''		'''''References'''''

	(1)Ruivo, A.; Gomes, C.; Lima, A.; Botelho, M. L.; Melo, R.; Belchior, A.; Pires de Matos, A. ''J. Cult. Herit.'' '''2008''', ''9'', e134-e137.		(1)Ruivo, A.; Gomes, C.; Lima, A.; Botelho, M. L.; Melo, R.; Belchior, A.; Pires de Matos, A. ''J. Cult. Herit.'' '''2008''', ''9'', e134-e137.

Cmditradmin at 21:55, 23 September 2009

2009-09-23T21:55:22Z

← Older revision		Revision as of 14:55, 23 September 2009
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	As can be seen from Equation 1, the extinction cross-section of a particle depends on the dielectric function of the metal of which the particle is composed. This gives rise to very different absorption and scattering characteristics for different metal nanoparticles. The maximum of ''C<sub>ext''</sub>(λ), or the resonance condition, will take place when the denominator of the right-hand side of the equation becomes minimal. This is fulfilled approximately at the wavelength ''λ<sub>p''</sub> for which ''ε'(λ<sub>p</sub>) = -2ε<sub>m''</sub>, if the imaginary part of the metal dielectric function, ''ε''(λ<sub>p</sub>)'' is small.<sup>8,15</sup> The frequency of the surface plasmon resonance, ''ω<sub>p''</sub>, is then observed at ''ω<sub>p''</sub> ''= c / λ<sub>p''</sub> where ''c'' is the speed of light in the medium. The frequency of the surface plasmon resonance, ''ω<sub>p''</sub>, is depicted in Figure 2 in terms of the period of the oscillation of the electric field and the conduction electrons within a metal nanoparticle. Mie theory and its dipolar approximation in the case of small particles can be successfully used to calculate extinction spectra of metal nanoparticles. Using an experimental determination of the dielectric function of the bulk metal and the particle radius, calculations based on Equation 1 yield extinction cross section often in excellent agreement with measured absorption spectra of the corresponding nanoparticles.<sup>17</sup>		As can be seen from Equation 1, the extinction cross-section of a particle depends on the dielectric function of the metal of which the particle is composed. This gives rise to very different absorption and scattering characteristics for different metal nanoparticles. The maximum of ''C<sub>ext''</sub>(λ), or the resonance condition, will take place when the denominator of the right-hand side of the equation becomes minimal. This is fulfilled approximately at the wavelength ''λ<sub>p''</sub> for which ''ε'(λ<sub>p</sub>) = -2ε<sub>m''</sub>, if the imaginary part of the metal dielectric function, ''ε''(λ<sub>p</sub>)'' is small.<sup>8,15</sup> The frequency of the surface plasmon resonance, ''ω<sub>p''</sub>, is then observed at ''ω<sub>p''</sub> ''= c / λ<sub>p''</sub> where ''c'' is the speed of light in the medium. The frequency of the surface plasmon resonance, ''ω<sub>p''</sub>, is depicted in Figure 2 in terms of the period of the oscillation of the electric field and the conduction electrons within a metal nanoparticle. Mie theory and its dipolar approximation in the case of small particles can be successfully used to calculate extinction spectra of metal nanoparticles. Using an experimental determination of the dielectric function of the bulk metal and the particle radius, calculations based on Equation 1 yield extinction cross section often in excellent agreement with measured absorption spectra of the corresponding nanoparticles.<sup>17</sup>

	The resonance condition implies that the surface plasmon resonance frequency depends heavily on the dielectric constant of the medium, ''?<sub>m''</sub>. Indeed, the color of colloidal gold changes with the dielectric constant of the solvent. Underwood and Mulvaney demonstrated that effect by measuring UV-Vis absorption spectra of polymer-stabilized gold nanoparticles of 16 nm in diameter in a series of solvents with different refractive indices. The authors showed that a change in the solvent refractive index from 1.30 to 1.60 lead to a shift in the measured maximum of the surface plasmon resonance band from 520 nm to 545 nm. Additionally, the measured positions of the maxima agreed perfectly with the values calculated with the use of Mie theory.<sup>14</sup>		The resonance condition implies that the surface plasmon resonance frequency depends heavily on the dielectric constant of the medium, ''ε<sub>m''</sub>. Indeed, the color of colloidal gold changes with the dielectric constant of the solvent. Underwood and Mulvaney demonstrated that effect by measuring UV-Vis absorption spectra of polymer-stabilized gold nanoparticles of 16 nm in diameter in a series of solvents with different refractive indices. The authors showed that a change in the solvent refractive index from 1.30 to 1.60 lead to a shift in the measured maximum of the surface plasmon resonance band from 520 nm to 545 nm. Additionally, the measured positions of the maxima agreed perfectly with the values calculated with the use of Mie theory.<sup>14</sup>

	From the perspective of a chemist it is interesting to ask how the absorption efficiency of gold nanoparticles compares with that of typical organic dyes. After all, the ''collective oscillation of the conduction band electrons'' in metal nanoparticles that gives rise to the surface-plasmon absorption band is a rather exotic concept to a chemist when put side-by-side with an absorption process in typical organic dyes, which often can be well-described as a ''one-electron transition'' between two molecular orbitals. Comparing experimentally determined molar extinction coefficients for gold nanoparticles with extinction coefficients of well-known dyes shows just how much more efficient an absorber a nanoparticle is when compared to an organic dye. As can be seen in Table 1, even for rather small nanoparticles (ca. 4 nm in diameter) the molar extinction coefficient is almost two orders of magnitude larger than that of Rhodamine B, which is considered to be an efficient absorber.		From the perspective of a chemist it is interesting to ask how the absorption efficiency of gold nanoparticles compares with that of typical organic dyes. After all, the ''collective oscillation of the conduction band electrons'' in metal nanoparticles that gives rise to the surface-plasmon absorption band is a rather exotic concept to a chemist when put side-by-side with an absorption process in typical organic dyes, which often can be well-described as a ''one-electron transition'' between two molecular orbitals. Comparing experimentally determined molar extinction coefficients for gold nanoparticles with extinction coefficients of well-known dyes shows just how much more efficient an absorber a nanoparticle is when compared to an organic dye. As can be seen in Table 1, even for rather small nanoparticles (ca. 4 nm in diameter) the molar extinction coefficient is almost two orders of magnitude larger than that of Rhodamine B, which is considered to be an efficient absorber.
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	[[Image:UV-vis_goldnano.jpg\|thumb\|300px\|'''Figure 3.''' UV-Vis absorption spectra acquired for toluene solutions of oleylamine-coated gold nanoparticles with different diameters. The spectra were normalized at 450 nm.]]		[[Image:UV-vis_goldnano.jpg\|thumb\|300px\|'''Figure 3.''' UV-Vis absorption spectra acquired for toluene solutions of oleylamine-coated gold nanoparticles with different diameters. The spectra were normalized at 450 nm.]]



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	===Gold Nanoparticles – Surface Plasmon Resonance Dynamics===		===Gold Nanoparticles – Surface Plasmon Resonance Dynamics===

	The large absorption cross-section values of the surface-plasmon resonance band imply that a NP is able to efficiently acquire a vast amount of energy when irradiated with light at the appropriate wavelength. Thus, it is interesting to ask questions about the excited-state deactivation pathways and their corresponding dynamics in gold nanoparticles. In other words, how efficient is the deactivation of a photoexcited gold nanoparticle?It is useful to clarify what is meant by excited-state deactivation in the context of the surface plasmon resonance in metal nanoparticles. Because the surface plasmon resonance is a manifestation of a ''coherent oscillation'' of the conduction band electrons, the loss of coherence is a form of deactivation of the excited state and it may have observable effects. However, this loss of coherence does not involve any energy redistribution, but merely the change of the plane in which each electron oscillates (i.e. the change of the plasmon wave vector), thus leading to the loss in coherence. This process is very fast, on the order of a few femtoseconds. More on this can be found in Ref.(8)<ref>Link, S.; El-Sayed, M. A. ''Annu. Rev. Phys. Chem.'' '''2003''', ''54'', 331-366.</ref> We are instead interested in the deactivation of the NP excited state via energy dissipation and the discussion in this section focuses on this process only.		The large absorption cross-section values of the surface-plasmon resonance band imply that a NP is able to efficiently acquire a vast amount of energy when irradiated with light at the appropriate wavelength. Thus, it is interesting to ask questions about the excited-state deactivation pathways and their corresponding dynamics in gold nanoparticles. In other words, how efficient is the deactivation of a photoexcited gold nanoparticle? It is useful to clarify what is meant by excited-state deactivation in the context of the surface plasmon resonance in metal nanoparticles. Because the surface plasmon resonance is a manifestation of a ''coherent oscillation'' of the conduction band electrons, the loss of coherence is a form of deactivation of the excited state and it may have observable effects. However, this loss of coherence does not involve any energy redistribution, but merely the change of the plane in which each electron oscillates (i.e. the change of the plasmon wave vector), thus leading to the loss in coherence. This process is very fast, on the order of a few femtoseconds. More on this can be found in Ref.(8)<ref>Link, S.; El-Sayed, M. A. ''Annu. Rev. Phys. Chem.'' '''2003''', ''54'', 331-366.</ref> We are instead interested in the deactivation of the NP excited state via energy dissipation and the discussion in this section focuses on this process only.


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	:<math>k_{FRET}=\frac {c \phi _{D} ^{fl} k^2} {n^4 \tau_D r_{(D-A)}^6} \int^{\infty}_{0} f_D (\nu) \varepsilon_A (\nu)\frac {d\nu}{\nu^4}\,\!</math> '''Equation 2'''		:<math>k_{FRET}=\frac {c \phi _{D} ^{fl} k^2} {n^4 \tau_D r_{(D-A)}^6} \int^{\infty}_{0} f_D (\nu) \varepsilon_A (\nu)\frac {d\nu}{\nu^4}\,\!</math> '''Equation 2'''

	where ''c'' is speed of light, ''k'' is an orientational factor defining the projection of angles between the transition dipole moment of a donor and that of an acceptor, and ''?'' is frequency. As already mentioned, the fact that gold nanoparticles have very large molar extinction coefficients makes them potentially excellent energy acceptors, according to Equation 2. A sizeable effort has been directed at studying promising fluorescent energy donor – gold nanoparticle systems in order to understand the process of energy transfer in these hybrid systems.		where
			''c'' is speed of light,
			''k'' is an orientational factor defining the projection of angles between the transition dipole moment of a donor and that of an acceptor, and
			''\nu'' is frequency. As already mentioned, the fact that gold nanoparticles have very large molar extinction coefficients makes them potentially excellent energy acceptors, according to Equation 2. A sizeable effort has been directed at studying promising fluorescent energy donor – gold nanoparticle systems in order to understand the process of energy transfer in these hybrid systems.

	Aguila and Murray showed that the fluorescence of dansyl cadaverine – a green-fluorescent dye – attached to the surface of small gold nanoparticles (1.6 nm in diameter) is quenched with respect to isolated dyes in solution.<sup>46</sup> The authors reported that the intensity of the fluorescence emission of the dye attached to the nanoparticles is two orders of magnitude less intense than the fluorescence of the free dye at same concentration.<sup>46</sup> This behavior was explained in terms of energy transfer from the photoexcited fluorophore to the metallic core of the gold nanoparticle. Additionally, the authors pointed out that the length of the alkyl-chain linker between the dansyl moiety and the nanoparticle surface had an influence on the fluorescence quenching efficiency, a more efficient quenching being observed when the linker was shorter, qualitatively consistent with the FRET model.<sup>46</sup>		Aguila and Murray showed that the fluorescence of dansyl cadaverine – a green-fluorescent dye – attached to the surface of small gold nanoparticles (1.6 nm in diameter) is quenched with respect to isolated dyes in solution.<sup>46</sup> The authors reported that the intensity of the fluorescence emission of the dye attached to the nanoparticles is two orders of magnitude less intense than the fluorescence of the free dye at same concentration.<sup>46</sup> This behavior was explained in terms of energy transfer from the photoexcited fluorophore to the metallic core of the gold nanoparticle. Additionally, the authors pointed out that the length of the alkyl-chain linker between the dansyl moiety and the nanoparticle surface had an influence on the fluorescence quenching efficiency, a more efficient quenching being observed when the linker was shorter, qualitatively consistent with the FRET model.<sup>46</sup>
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	:<math>\phi_{fl} = \frac {k_{rad}} {k_{rad} + k_{nr}}\,\!</math> ,'''Equation 3'''		:<math>\phi_{fl} = \frac {k_{rad}} {k_{rad} + k_{nr}}\,\!</math> ,'''Equation 3'''

	where ''k<sub>nr''</sub> is a combined rate of all processes leading to a nonradiative deactivation of the excited state of the dye. For simple solutions of fluorescent molecules the radiative rate, ''k<sub>rad''</sub>, remains more or less unchanged, thus usually fluorescence quenching is not attributed to the changes in the natural fluorescence lifetime, ?<sub>rad</sub> = ''1/k<sub>rad''</sub>. Dulkeith et al. showed that depending on the size of the gold nanoparticles, 1 – 30 nm in diameter in their studies, the radiative rate of particle-attached lissamine molecules was decreased by as much as two orders of magnitude with respect to free dye molecules.<sup>44</sup> At the same time the nonradiative decay rate, ''k<sub>nr''</sub> in Equation 3, increased by almost two orders of magnitude for the studied systems. Thus, according to Equation 3 the decrease in the fluorescence quantum yield was caused by both the suppression of the radiative rate and the increase of the nonradiative rate.<sup>44</sup> Dulkeith et al. later investigated the photophysics of a red-fluorescent cyanine molecule (Cy5) attached to gold nanoparticles with a diameter of 6 nm via rigid DNA linkers.<sup>53</sup> Again, fluorescence quenching of the dye was observed after attaching it to the surface on nanoparticles via linkers of different lengths, and the radiative rates of the dye were suppressed in all of the studied systems. In the studied range of dye-nanoparticle distances, 2 nm to 16 nm, the nonradiative rate, which was most likely dominated by energy transfer, seemed to be increased only for systems incorporating the shortest DNA linkers. For systems with the dye-nanoparticle distance larger than 4 nm the nonradiative rate seemed to be the same as that for free dye and the fluorescence quenching was governed solely by radiative rate suppression, according to Equation 3.<sup>53</sup>		where ''k<sub>nr''</sub> is a combined rate of all processes leading to a nonradiative deactivation of the excited state of the dye. For simple solutions of fluorescent molecules the radiative rate, ''k<sub>rad''</sub>, remains more or less unchanged, thus usually fluorescence quenching is not attributed to the changes in the natural fluorescence lifetime, τ<sub>rad</sub> = ''1/k<sub>rad''</sub>. Dulkeith et al. showed that depending on the size of the gold nanoparticles, 1 – 30 nm in diameter in their studies, the radiative rate of particle-attached lissamine molecules was decreased by as much as two orders of magnitude with respect to free dye molecules.<sup>44</sup> At the same time the nonradiative decay rate, ''k<sub>nr''</sub> in Equation 3, increased by almost two orders of magnitude for the studied systems. Thus, according to Equation 3 the decrease in the fluorescence quantum yield was caused by both the suppression of the radiative rate and the increase of the nonradiative rate.<sup>44</sup> Dulkeith et al. later investigated the photophysics of a red-fluorescent cyanine molecule (Cy5) attached to gold nanoparticles with a diameter of 6 nm via rigid DNA linkers.<sup>53</sup> Again, fluorescence quenching of the dye was observed after attaching it to the surface on nanoparticles via linkers of different lengths, and the radiative rates of the dye were suppressed in all of the studied systems. In the studied range of dye-nanoparticle distances, 2 nm to 16 nm, the nonradiative rate, which was most likely dominated by energy transfer, seemed to be increased only for systems incorporating the shortest DNA linkers. For systems with the dye-nanoparticle distance larger than 4 nm the nonradiative rate seemed to be the same as that for free dye and the fluorescence quenching was governed solely by radiative rate suppression, according to Equation 3.<sup>53</sup>

	The radiative rate modification by metal spheres has been described theoretically by Gersten and Nitzan.<sup>54</sup> Using a purely classical description of an oscillating point dipole placed in close proximity of a metallic sphere with a known dielectric function, the authors calculated the effective radiative and nonradiative rate of the radiating dipole. Dipole – particle distance, particle diameter, dipole orientation with respect to the particle surface, and the dipole-oscillation frequency were found to be important parameters influencing the final values of the radiative and nonradiative rates.<sup>54</sup>		The radiative rate modification by metal spheres has been described theoretically by Gersten and Nitzan.<sup>54</sup> Using a purely classical description of an oscillating point dipole placed in close proximity of a metallic sphere with a known dielectric function, the authors calculated the effective radiative and nonradiative rate of the radiating dipole. Dipole – particle distance, particle diameter, dipole orientation with respect to the particle surface, and the dipole-oscillation frequency were found to be important parameters influencing the final values of the radiative and nonradiative rates.<sup>54</sup>

Cmditradmin: /* Energy Transfer to the Nanoparticle and Radiative Rate Modification */

2009-09-23T17:54:28Z

Energy Transfer to the Nanoparticle and Radiative Rate Modification

← Older revision		Revision as of 10:54, 23 September 2009
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	===Energy Transfer to the Nanoparticle and Radiative Rate Modification===		===Energy Transfer to the Nanoparticle and Radiative Rate Modification===

	Energy transfer from fluorescent organic dyes to gold nanoparticles is generally considered to be the major process leading to the excited-state deactivation of the dyes.<sup>45</sup> Before introducing examples of published reports on the energy transfer involving gold nanoparticles, it is useful to discuss the mechanisms by which the process is likely to occur. Perhaps the most familiar mechanism is that of energy transfer via dipole-dipole interactions. Named after the scientist who described the mechanism mathematically, the rate of Förster resonance energy transfer (FRET) from an energy donor to an energy acceptor, ''k<sub>FRET''</sub>, depends on: the fluorescence quantum yield of the donor, φ<sub>D</sub><sup>fl</sup>; its excited state lifetime,τ<sub>D</sub>; refractive index, ''n''; the distance between the donor and the acceptor, r<sub>D-A</sub>; and the spectral overlap of the fluorescence of the donor, ''f<sub>D</sub>(ν)'' with the molar extinction coefficient of the acceptor, ''&~~varepsilon~~;<sub>A</sub>(ν)''. The rate constant is described by the following equation:<sup>17</sup>		Energy transfer from fluorescent organic dyes to gold nanoparticles is generally considered to be the major process leading to the excited-state deactivation of the dyes.<sup>45</sup> Before introducing examples of published reports on the energy transfer involving gold nanoparticles, it is useful to discuss the mechanisms by which the process is likely to occur. Perhaps the most familiar mechanism is that of energy transfer via dipole-dipole interactions. Named after the scientist who described the mechanism mathematically, the rate of Förster resonance energy transfer (FRET) from an energy donor to an energy acceptor, ''k<sub>FRET''</sub>, depends on: the fluorescence quantum yield of the donor, φ<sub>D</sub><sup>fl</sup>; its excited state lifetime,τ<sub>D</sub>; refractive index, ''n''; the distance between the donor and the acceptor, r<sub>D-A</sub>; and the spectral overlap of the fluorescence of the donor, ''f<sub>D</sub>(ν)'' with the molar extinction coefficient of the acceptor, ''ε<sub>A</sub>(ν)''. The rate constant is described by the following equation:<sup>17</sup>

	:<math>k_{FRET}=\frac {c \phi _{D} ^{fl} k^2} {n^4 \tau_D r_{(D-A)}^6} \int^{\infty}_{0} f_D (\nu) \varepsilon_A (\nu)\frac {d\nu}{\nu^4}\,\!</math> '''Equation 2'''		:<math>k_{FRET}=\frac {c \phi _{D} ^{fl} k^2} {n^4 \tau_D r_{(D-A)}^6} \int^{\infty}_{0} f_D (\nu) \varepsilon_A (\nu)\frac {d\nu}{\nu^4}\,\!</math> '''Equation 2'''

Cmditradmin: /* Energy Transfer to the Nanoparticle and Radiative Rate Modification */

2009-09-23T17:54:02Z

Energy Transfer to the Nanoparticle and Radiative Rate Modification

← Older revision		Revision as of 10:54, 23 September 2009
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	===Energy Transfer to the Nanoparticle and Radiative Rate Modification===		===Energy Transfer to the Nanoparticle and Radiative Rate Modification===

	Energy transfer from fluorescent organic dyes to gold nanoparticles is generally considered to be the major process leading to the excited-state deactivation of the dyes.<sup>45</sup> Before introducing examples of published reports on the energy transfer involving gold nanoparticles, it is useful to discuss the mechanisms by which the process is likely to occur. Perhaps the most familiar mechanism is that of energy transfer via dipole-dipole interactions. Named after the scientist who described the mechanism mathematically, the rate of Förster resonance energy transfer (FRET) from an energy donor to an energy acceptor, ''k<sub>FRET''</sub>, depends on: the fluorescence quantum yield of the donor, φ<sub>D</sub><sup>fl</sup~~><nowiki~~>; its excited state lifetime,τ<sub>D</sub>; refractive index, ''n''; the distance between the donor and the acceptor, r<sub>D-A</sub>; and the spectral overlap of the fluorescence of the donor, ''f<sub>D</sub>(ν)'' with the molar extinction coefficient of the acceptor, ''ϵ<sub>A</sub>(ν)''. The rate constant is described by the following equation:<sup>17</sup>		Energy transfer from fluorescent organic dyes to gold nanoparticles is generally considered to be the major process leading to the excited-state deactivation of the dyes.<sup>45</sup> Before introducing examples of published reports on the energy transfer involving gold nanoparticles, it is useful to discuss the mechanisms by which the process is likely to occur. Perhaps the most familiar mechanism is that of energy transfer via dipole-dipole interactions. Named after the scientist who described the mechanism mathematically, the rate of Förster resonance energy transfer (FRET) from an energy donor to an energy acceptor, ''k<sub>FRET''</sub>, depends on: the fluorescence quantum yield of the donor, φ<sub>D</sub><sup>fl</sup>; its excited state lifetime,τ<sub>D</sub>; refractive index, ''n''; the distance between the donor and the acceptor, r<sub>D-A</sub>; and the spectral overlap of the fluorescence of the donor, ''f<sub>D</sub>(ν)'' with the molar extinction coefficient of the acceptor, ''ϵ<sub>A</sub>(ν)''. The rate constant is described by the following equation:<sup>17</sup>

	:<math>k_{FRET}=\frac {c \phi _{D} ^{fl} k^2} {n^4 \tau_D r_{(D-A)}^6} \int^{\infty}_{0} f_D (\nu) \varepsilon_A (\nu)\frac {d\nu}{\nu^4}\,\!</math> '''Equation 2'''		:<math>k_{FRET}=\frac {c \phi _{D} ^{fl} k^2} {n^4 \tau_D r_{(D-A)}^6} \int^{\infty}_{0} f_D (\nu) \varepsilon_A (\nu)\frac {d\nu}{\nu^4}\,\!</math> '''Equation 2'''

Cmditradmin: /* Energy Transfer to the Nanoparticle and Radiative Rate Modification */

2009-09-23T17:53:32Z

Energy Transfer to the Nanoparticle and Radiative Rate Modification

← Older revision		Revision as of 10:53, 23 September 2009
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	===Energy Transfer to the Nanoparticle and Radiative Rate Modification===		===Energy Transfer to the Nanoparticle and Radiative Rate Modification===

	Energy transfer from fluorescent organic dyes to gold nanoparticles is generally considered to be the major process leading to the excited-state deactivation of the dyes.<sup>45</sup> Before introducing examples of published reports on the energy transfer involving gold nanoparticles, it is useful to discuss the mechanisms by which the process is likely to occur. Perhaps the most familiar mechanism is that of energy transfer via dipole-dipole interactions. Named after the scientist who described the mechanism mathematically, the rate of Förster resonance energy transfer (FRET) from an energy donor to an energy acceptor, ''k<sub>FRET''</sub>, depends on: the fluorescence quantum yield of the donor, ~~?Dfl~~<nowiki>; its excited state lifetime, <~~/nowiki~~>?D<~~nowiki~~>; refractive index, ~~</nowiki>~~''n''~~<nowiki>~~; the distance between the donor and the acceptor, <~~/nowiki~~>rD-A<~~nowiki~~>; and the spectral overlap of the fluorescence of the donor, ~~</nowiki>~~''f<sub>D</sub>(?)'' with the molar extinction coefficient of the acceptor, ''?<sub>A</sub>(?)''. The rate constant is described by the following equation:<sup>17</sup>		Energy transfer from fluorescent organic dyes to gold nanoparticles is generally considered to be the major process leading to the excited-state deactivation of the dyes.<sup>45</sup> Before introducing examples of published reports on the energy transfer involving gold nanoparticles, it is useful to discuss the mechanisms by which the process is likely to occur. Perhaps the most familiar mechanism is that of energy transfer via dipole-dipole interactions. Named after the scientist who described the mechanism mathematically, the rate of Förster resonance energy transfer (FRET) from an energy donor to an energy acceptor, ''k<sub>FRET''</sub>, depends on: the fluorescence quantum yield of the donor, φ<sub>D</sub><sup>fl</sup><nowiki>; its excited state lifetime,τ<sub>D</sub>; refractive index, ''n''; the distance between the donor and the acceptor, r<sub>D-A</sub>; and the spectral overlap of the fluorescence of the donor, ''f<sub>D</sub>(ν)'' with the molar extinction coefficient of the acceptor, ''ϵ<sub>A</sub>(ν)''. The rate constant is described by the following equation:<sup>17</sup>

	:<math>k_{FRET}=\frac {c \phi _{D} ^{fl} k^2} {n^4 \tau_D r_{(D-A)}^6} \int^{\infty}_{0} f_D (\nu) \varepsilon_A (\nu)\frac {d\nu}{\nu^4}\,\!</math> '''Equation 2'''		:<math>k_{FRET}=\frac {c \phi _{D} ^{fl} k^2} {n^4 \tau_D r_{(D-A)}^6} \int^{\infty}_{0} f_D (\nu) \varepsilon_A (\nu)\frac {d\nu}{\nu^4}\,\!</math> '''Equation 2'''
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	(65)Chen, S.; Templeton, A. C.; Murray, R. W. ''Langmuir'' '''2000''', ''16'', 3543-3548.		(65)Chen, S.; Templeton, A. C.; Murray, R. W. ''Langmuir'' '''2000''', ''16'', 3543-3548.



	== References ==		== References ==

	<references/>		<references/>

Cmditradmin at 17:44, 23 September 2009

2009-09-23T17:44:49Z

← Older revision		Revision as of 10:44, 23 September 2009
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	===Gold Nanoparticles – Historical Perspective===		===Gold Nanoparticles – Historical Perspective===

	Metal nanoparticles show very interesting optical properties. The use of these miniscule objects for glass staining dates back to the ancient times. “Ruby glass”, which is essentially glass containing gold nanoparticles, has been used since antiquity until the present.~~<sup>1</sup>~~ A classic example of an ancient piece of art gaining its appeal from the color produced by metal nanoparticles is the late Roman “Lycurgus Cup”, which is exhibited in the British Museum.<~~sup~~>1-3</~~sup~~> Depicting the mythological scene of Lycurgus’s entrapment by the vine-turned Ambrosia, a maenad of Dionysus, the cup shows extraordinary dichroic behavior exhibiting red color in transmission and green color in reflection (see Figure 1). This beautiful effect is due to absorption and scattering of gold and silver nanoparticles which are present in the glass from which the cup is made.<ref>Ruivo, A.; Gomes, C.; Lima, A.; Botelho, M. L.; Melo, R.; Belchior, A.; Pires de Matos, A. J. Cult. Herit. 2008, 9, e134-e137.</ref><ref>Wagner, F. E.; Haslbeck, S.; Stievano, L.; Calogero, S.; Pankhurst, Q. A.; Martinek, P. Nature 2000, 407, 691-692 <sup>1,2'''</ref~~></sup~~>		Metal nanoparticles show very interesting optical properties. The use of these miniscule objects for glass staining dates back to the ancient times. “Ruby glass”, which is essentially glass containing gold nanoparticles, has been used since antiquity until the present. A classic example of an ancient piece of art gaining its appeal from the color produced by metal nanoparticles is the late Roman “Lycurgus Cup”, which is exhibited in the British Museum.<ref>Daniel, M. C.; Astruc, D. Chemical Reviews (Washington, DC, United States) 2004, 104, 293-346.</ref> Depicting the mythological scene of Lycurgus’s entrapment by the vine-turned Ambrosia, a maenad of Dionysus, the cup shows extraordinary dichroic behavior exhibiting red color in transmission and green color in reflection (see Figure 1). This beautiful effect is due to absorption and scattering of gold and silver nanoparticles which are present in the glass from which the cup is made.<ref>Ruivo, A.; Gomes, C.; Lima, A.; Botelho, M. L.; Melo, R.; Belchior, A.; Pires de Matos, A. J. Cult. Herit. 2008, 9, e134-e137.</ref><ref>Wagner, F. E.; Haslbeck, S.; Stievano, L.; Calogero, S.; Pankhurst, Q. A.; Martinek, P. Nature 2000, 407, 691-692 <sup>1,2'''</ref>


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	A revolution in the use of gold metal for glass and ceramic staining did not take place until late 17<sup>th</sup> century when it was discovered that combining aqua regia solution of gold and tin produces a precipitate with deep and vibrant red color.<~~sup~~>4</~~sup~~> Named “purple of Cassius”, after its alleged inventor, the colorant became one of the most successful red pigments used in the production of glass and ceramics, and it is still in use today.<sup>4,5'''</sup>,''' Even though Andreas Cassius had been given the credit of the discovery and the 1685 recipe of the “purple of Cassius” he was not the first to discover the recipe for the preparation of the famous colorant. By 1659 a report of the preparation of the red pigment had already been published by Johann Rudolf Glauber, and by about 1679 the colorant had been in use in a glass factory at Potsdam. The history of this is rather fascinating as there were apparently two persons named Andreas Cassius – a father and the son to whom the discovery of “purple of Cassius” was attributed – and it is not clear what the role of each one of them was in the development of the dye. More on this can be found in Ref.(5)<ref>Hunt, L. ''Gold Bull.'' 1976, ''9'', 134-39</ref>.'''		A revolution in the use of gold metal for glass and ceramic staining did not take place until late 17<sup>th</sup> century when it was discovered that combining aqua regia solution of gold and tin produces a precipitate with deep and vibrant red color.<ref>Carbert, J. Gold Bull. 1980, 13, 144-50.</ref> Named “purple of Cassius”, after its alleged inventor, the colorant became one of the most successful red pigments used in the production of glass and ceramics, and it is still in use today.<sup>4,5'''</sup>,''' Even though Andreas Cassius had been given the credit of the discovery and the 1685 recipe of the “purple of Cassius” he was not the first to discover the recipe for the preparation of the famous colorant. By 1659 a report of the preparation of the red pigment had already been published by Johann Rudolf Glauber, and by about 1679 the colorant had been in use in a glass factory at Potsdam. The history of this is rather fascinating as there were apparently two persons named Andreas Cassius – a father and the son to whom the discovery of “purple of Cassius” was attributed – and it is not clear what the role of each one of them was in the development of the dye. More on this can be found in Ref.(5)<ref>Hunt, L. ''Gold Bull.'' 1976, ''9'', 134-39</ref>.'''

	Apart from their use for staining glass and ceramics, gold particles were also employed in photography. In 1842 Sir John Herschel developed his “chrysotype” process in which a photochemically reduced iron(II) salt was exposed to a tetrachloroaurate(III) salt, resulting in the reduction of gold(III) to gold(0), with subsequent formation of gold particles.<~~sup~~>6</~~sup~~> Even though this method did not survive competition from the silver photography invented earlier by Talbot, gold particles were successfully and routinely used for toning silver photographs, which gave them increased stability.<~~sup~~>6</~~sup~~> '''		Apart from their use for staining glass and ceramics, gold particles were also employed in photography. In 1842 Sir John Herschel developed his “chrysotype” process in which a photochemically reduced iron(II) salt was exposed to a tetrachloroaurate(III) salt, resulting in the reduction of gold(III) to gold(0), with subsequent formation of gold particles.<ref>Ware, M. Gold Bull 2006, 39, 124-131.</ref> Even though this method did not survive competition from the silver photography invented earlier by Talbot, gold particles were successfully and routinely used for toning silver photographs, which gave them increased stability.<ref>Ware, M. Gold Bull 2006, 39, 124-131.</ref> '''

	Much work in the early days of chemistry was stimulated by the “purple of Cassius”. The question of the composition and nature of the dye constituted a serious, often raucously argued, scientific problem in the 19<sup>th</sup> century. The first time it was recognized that “purple of Cassius” is composed of ''“extremely finely divided gold”'' was in 1802. This was published by Jeremias Benjamin Richter, the same chemist who bequeathed us the law of definite proportions. A number of historically famous chemists took part in the discussion – Joseph Proust, Jöns Jacob Berzelius, and Joseph Louis Gay-Lussac, just to name a few. The main argument in those days revolved around the question whether the gold in “purple of Cassius” is in the form of an oxide or in its metallic form.<sup>4</sup> In the contemporary literature the moment considered as the definitive recognition of gold particles as the main constituents of “purple of Cassius” seems to be the Bakerian Lecture Michael Faraday gave to the Royal Society in 1857.<sup>7</sup> Faraday devoted a part of his scientific career to studying the interactions of ''“finely divided”'' metals, especially gold, with light by which, as his writing implies, he was fascinated. It is captivating to read about his motivation to study the metal particles in the context of these interactions. Being an exceptional experimentalist, Faraday intuitively felt that experiments with highly absorbing metal particles with sizes smaller than the wavelength of light may give useful insights supporting the theory of ether, which he accepted.<sup>7</sup> Though the legacy of Faraday’s work obviously does not include a proof for the existence of ether, the body of his work on the interaction of metal particles with light marks the beginning of modern spectroscopy research on colloidal gold.<sup>3,8'''</sup>		Much work in the early days of chemistry was stimulated by the “purple of Cassius”. The question of the composition and nature of the dye constituted a serious, often raucously argued, scientific problem in the 19<sup>th</sup> century. The first time it was recognized that “purple of Cassius” is composed of ''“extremely finely divided gold”'' was in 1802. This was published by Jeremias Benjamin Richter, the same chemist who bequeathed us the law of definite proportions. A number of historically famous chemists took part in the discussion – Joseph Proust, Jöns Jacob Berzelius, and Joseph Louis Gay-Lussac, just to name a few. The main argument in those days revolved around the question whether the gold in “purple of Cassius” is in the form of an oxide or in its metallic form.<sup>4</sup> In the contemporary literature the moment considered as the definitive recognition of gold particles as the main constituents of “purple of Cassius” seems to be the Bakerian Lecture Michael Faraday gave to the Royal Society in 1857.<sup>7</sup> Faraday devoted a part of his scientific career to studying the interactions of ''“finely divided”'' metals, especially gold, with light by which, as his writing implies, he was fascinated. It is captivating to read about his motivation to study the metal particles in the context of these interactions. Being an exceptional experimentalist, Faraday intuitively felt that experiments with highly absorbing metal particles with sizes smaller than the wavelength of light may give useful insights supporting the theory of ether, which he accepted.<sup>7</sup> Though the legacy of Faraday’s work obviously does not include a proof for the existence of ether, the body of his work on the interaction of metal particles with light marks the beginning of modern spectroscopy research on colloidal gold.<sup>3,8'''</sup>

Cmditradmin at 17:42, 23 September 2009

2009-09-23T17:42:42Z

← Older revision		Revision as of 10:42, 23 September 2009
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	===Gold Nanoparticles – Historical Perspective===		===Gold Nanoparticles – Historical Perspective===

	Metal nanoparticles show very interesting optical properties. The use of these miniscule objects for glass staining dates back to the ancient times. “Ruby glass”, which is essentially glass containing gold nanoparticles, has been used since antiquity until the present.<sup>1</sup> A classic example of an ancient piece of art gaining its appeal from the color produced by metal nanoparticles is the late Roman “Lycurgus Cup”, which is exhibited in the British Museum.<sup>1-3</sup> Depicting the mythological scene of Lycurgus’s entrapment by the vine-turned Ambrosia, a maenad of Dionysus, the cup shows extraordinary dichroic behavior exhibiting red color in transmission and green color in reflection (see Figure 1). This beautiful effect is due to absorption and scattering of gold and silver nanoparticles which are present in the glass from which the cup is made.<sup>1,2'''</sup>		Metal nanoparticles show very interesting optical properties. The use of these miniscule objects for glass staining dates back to the ancient times. “Ruby glass”, which is essentially glass containing gold nanoparticles, has been used since antiquity until the present.<sup>1</sup> A classic example of an ancient piece of art gaining its appeal from the color produced by metal nanoparticles is the late Roman “Lycurgus Cup”, which is exhibited in the British Museum.<sup>1-3</sup> Depicting the mythological scene of Lycurgus’s entrapment by the vine-turned Ambrosia, a maenad of Dionysus, the cup shows extraordinary dichroic behavior exhibiting red color in transmission and green color in reflection (see Figure 1). This beautiful effect is due to absorption and scattering of gold and silver nanoparticles which are present in the glass from which the cup is made.<ref>Ruivo, A.; Gomes, C.; Lima, A.; Botelho, M. L.; Melo, R.; Belchior, A.; Pires de Matos, A. J. Cult. Herit. 2008, 9, e134-e137.</ref><ref>Wagner, F. E.; Haslbeck, S.; Stievano, L.; Calogero, S.; Pankhurst, Q. A.; Martinek, P. Nature 2000, 407, 691-692 <sup>1,2'''</ref></sup>

Cmditradmin: /* Gold Nanoparticles – Surface Plasmon Resonance Dynamics */

2009-09-23T17:40:36Z

Gold Nanoparticles – Surface Plasmon Resonance Dynamics

← Older revision		Revision as of 10:40, 23 September 2009
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	===Gold Nanoparticles – Surface Plasmon Resonance Dynamics===		===Gold Nanoparticles – Surface Plasmon Resonance Dynamics===

	The large absorption cross-section values of the surface-plasmon resonance band imply that a NP is able to efficiently acquire a vast amount of energy when irradiated with light at the appropriate wavelength. Thus, it is interesting to ask questions about the excited-state deactivation pathways and their corresponding dynamics in gold nanoparticles. In other words, how efficient is the deactivation of a photoexcited gold nanoparticle?It is useful to clarify what is meant by excited-state deactivation in the context of the surface plasmon resonance in metal nanoparticles. Because the surface plasmon resonance is a manifestation of a ''coherent oscillation'' of the conduction band electrons, the loss of coherence is a form of deactivation of the excited state and it may have observable effects. However, this loss of coherence does not involve any energy redistribution, but merely the change of the plane in which each electron oscillates (i.e. the change of the plasmon wave vector), thus leading to the loss in coherence. This process is very fast, on the order of a few femtoseconds. More on this can be found in Ref.(8)Link, S.; El-Sayed, M. A. ''Annu. Rev. Phys. Chem.'' '''2003''', ''54'', 331-366. We are instead interested in the deactivation of the NP excited state via energy dissipation and the discussion in this section focuses on this process only.		The large absorption cross-section values of the surface-plasmon resonance band imply that a NP is able to efficiently acquire a vast amount of energy when irradiated with light at the appropriate wavelength. Thus, it is interesting to ask questions about the excited-state deactivation pathways and their corresponding dynamics in gold nanoparticles. In other words, how efficient is the deactivation of a photoexcited gold nanoparticle?It is useful to clarify what is meant by excited-state deactivation in the context of the surface plasmon resonance in metal nanoparticles. Because the surface plasmon resonance is a manifestation of a ''coherent oscillation'' of the conduction band electrons, the loss of coherence is a form of deactivation of the excited state and it may have observable effects. However, this loss of coherence does not involve any energy redistribution, but merely the change of the plane in which each electron oscillates (i.e. the change of the plasmon wave vector), thus leading to the loss in coherence. This process is very fast, on the order of a few femtoseconds. More on this can be found in Ref.(8)<ref>Link, S.; El-Sayed, M. A. ''Annu. Rev. Phys. Chem.'' '''2003''', ''54'', 331-366.</ref> We are instead interested in the deactivation of the NP excited state via energy dissipation and the discussion in this section focuses on this process only.

Cmditradmin: /* Gold Nanoparticles – Surface Plasmon Resonance Dynamics */

2009-09-23T17:40:10Z

Gold Nanoparticles – Surface Plasmon Resonance Dynamics

← Older revision		Revision as of 10:40, 23 September 2009
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	A second process responsible for the equilibration of the photoexcited electrons in the Au NP after photoexcitation is electron-electron relaxation. The high-energy electrons after the photoexcitation can undergo collisions with other electrons present in the volume of the nanoparticle, leading to the partition of the energy between the electrons. This process leads to a change in the energetic distribution of electrons from the highly non-thermal distribution present immediately after the photoexcitation to a statistical Fermi-Dirac distribution with a high-temperature Fermi level.Sun et al. found experimentally that in bulk gold this thermalization process is ultrafast, reporting a time constant of ca. 0.5 ps.<sup>26</sup>		A second process responsible for the equilibration of the photoexcited electrons in the Au NP after photoexcitation is electron-electron relaxation. The high-energy electrons after the photoexcitation can undergo collisions with other electrons present in the volume of the nanoparticle, leading to the partition of the energy between the electrons. This process leads to a change in the energetic distribution of electrons from the highly non-thermal distribution present immediately after the photoexcitation to a statistical Fermi-Dirac distribution with a high-temperature Fermi level.Sun et al. found experimentally that in bulk gold this thermalization process is ultrafast, reporting a time constant of ca. 0.5 ps.<sup>26</sup>

	Since the particle is metallic, there is essentially a continuum of states available for the electrons and it is useful to talk about the electrons in terms of their kinetic energy / temperature. However, this is true only for particles larger than roughly 2 nm, as for particles with smaller sizes the molecular-like behavior starts dominating and size-induced gaps appear in the density of states of these systems. More on this can be found in the work of Prof. El-Sayed and Prof. Whetten and their research groups at Georgia Tech. For example see Ref. (25)Logunov, S. L.; Ahmadi, T. S.; El-Sayed, M. A.; Khoury, J. T.; Whetten, R. L. ''J. Phys. Chem. B'' '''1997''', ''101'', 3713-3719.		Since the particle is metallic, there is essentially a continuum of states available for the electrons and it is useful to talk about the electrons in terms of their kinetic energy / temperature. However, this is true only for particles larger than roughly 2 nm, as for particles with smaller sizes the molecular-like behavior starts dominating and size-induced gaps appear in the density of states of these systems. More on this can be found in the work of Prof. El-Sayed and Prof. Whetten and their research groups at Georgia Tech. For example see Ref. (25)<ref>Logunov, S. L.; Ahmadi, T. S.; El-Sayed, M. A.; Khoury, J. T.; Whetten, R. L. ''J. Phys. Chem. B'' '''1997''', ''101'', 3713-3719.</ref>

	The “hot electrons” further lose their kinetic energy due to collisions with the ionic crystal lattice of the NP. This process, usually referred to as electron-phonon relaxation, leads to the lowering of the Fermi level and thermalization of the crystal lattice of the nanoparticle.<sup>8</sup> Hodak et al. showed that the electron-phonon-relaxation time constant in gold NPs did not depend on the size of nanoparticles for samples within the 2.5 nm to 120 nm size range,<sup>18</sup> and that its value (ca. 0.7 ps) was very similar to that measured for bulk gold films.<sup>26</sup> Since higher laser power causes a higher temperature jump of the electrons after electron-electron relaxation the electron-phonon-relaxation time constant depends on the power of the excitation beam, as shown experimentally by various researchers.<sup>8,18,22</sup> A spectacular phenomenon related to the heating of the crystal lattice of gold NPs due to the electron-phonon relaxation has been demonstrated by Hodak et al.<sup>27</sup> The researchers observed coherent oscillations in the signal of the surface-plasmon-resonance-band bleach in a femtosecond transient absorption experiment. The oscillations showed a frequency of ca. 6 cm<sup>-1</sup> and coherence time of ca. 15 ps, which was limited by the polydispersity of the sample. The authors attributed the observed phenomenon to a vibrational “breathing mode” of gold nanoparticles with the hot lattice induced coherently by the photoexcitation followed by electron-phonon relaxation.<sup>27</sup> The expanding – shrinking lattice of a nanoparticle causes shifts in the surface plasmon resonance band, thus causing the measured bleaching signal to show an oscillatory behavior.<sup>8,27</sup>		The “hot electrons” further lose their kinetic energy due to collisions with the ionic crystal lattice of the NP. This process, usually referred to as electron-phonon relaxation, leads to the lowering of the Fermi level and thermalization of the crystal lattice of the nanoparticle.<sup>8</sup> Hodak et al. showed that the electron-phonon-relaxation time constant in gold NPs did not depend on the size of nanoparticles for samples within the 2.5 nm to 120 nm size range,<sup>18</sup> and that its value (ca. 0.7 ps) was very similar to that measured for bulk gold films.<sup>26</sup> Since higher laser power causes a higher temperature jump of the electrons after electron-electron relaxation the electron-phonon-relaxation time constant depends on the power of the excitation beam, as shown experimentally by various researchers.<sup>8,18,22</sup> A spectacular phenomenon related to the heating of the crystal lattice of gold NPs due to the electron-phonon relaxation has been demonstrated by Hodak et al.<sup>27</sup> The researchers observed coherent oscillations in the signal of the surface-plasmon-resonance-band bleach in a femtosecond transient absorption experiment. The oscillations showed a frequency of ca. 6 cm<sup>-1</sup> and coherence time of ca. 15 ps, which was limited by the polydispersity of the sample. The authors attributed the observed phenomenon to a vibrational “breathing mode” of gold nanoparticles with the hot lattice induced coherently by the photoexcitation followed by electron-phonon relaxation.<sup>27</sup> The expanding – shrinking lattice of a nanoparticle causes shifts in the surface plasmon resonance band, thus causing the measured bleaching signal to show an oscillatory behavior.<sup>8,27</sup>
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	#		#

	===Noble Metal Nanoparticles – Local Field Effects===		===Noble Metal Nanoparticles – Local Field Effects===

Cmditradmin at 17:39, 23 September 2009

2009-09-23T17:39:19Z

← Older revision		Revision as of 10:39, 23 September 2009
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			== References ==

	~~----~~
	<references/>		<references/>