Fluorescent Lifetime Quenching near d = 1.5 nm Gold Nanoparticles:  Probing NSET Validity (original) (raw)

Abstract

The fluorescence behavior of molecular dyes at discrete distances from 1.5 nm diameter gold nanoparticles as a function of distance and energy is investigated. Photoluminescence and luminescence lifetime measurements both demonstrate quenching behavior consistent with 1/d 4 separation distance from dye to the surface of the nanoparticle. In agreement with the model of Persson and Lang, all experimental data show that energy transfer to the metal surface is the dominant quenching mechanism, and the radiative rate is unchanged throughout the experiment.

Figures (9)

The relationship between photoluminescence (PL) intensity and lifetimes is easily shown by considering that where eq 1 states that the observed lifetime, tops, is the inverse of all rates of decay, Kons; where Kops is the sum of the radiative rate, k,, the nonradia tive rate, kn, and the rate of energy transfer, ket. The radiative and nonradiative rates are normally considered constants for a dye under defined conditions, leaving ke, as the major contributor to rate of energy trans the shortening of an observed lifetime. The fer may be calculated, therefore, from the difference in rates of excited-state de-excitation in the presence and absence of gold quencher:

The relationship between photoluminescence (PL) intensity and lifetimes is easily shown by considering that where eq 1 states that the observed lifetime, tops, is the inverse of all rates of decay, Kons; where Kops is the sum of the radiative rate, k,, the nonradia tive rate, kn, and the rate of energy transfer, ket. The radiative and nonradiative rates are normally considered constants for a dye under defined conditions, leaving ke, as the major contributor to rate of energy trans the shortening of an observed lifetime. The fer may be calculated, therefore, from the difference in rates of excited-state de-excitation in the presence and absence of gold quencher:

Figure 1. Scheme of DNA binding to a 1.5 nm Au NP. By varying the length of the DNA strand, the terminal dye fluorophore is separated from the Au NP by discrete distances (168, 120, and 69 A)

Figure 1. Scheme of DNA binding to a 1.5 nm Au NP. By varying the length of the DNA strand, the terminal dye fluorophore is separated from the Au NP by discrete distances (168, 120, and 69 A)

Figure 2. Lifetimes of the 15 bp (green), 30 bp (red), and 45 bp (black) NP—dsDNA —dye assemblies for FAM (a) and for Cy5 (b) relative to dsDNA —dye controls (blue, top). The data have been normalized and offset vertically for viewing. Single-exponential fits through the data are shown (—).

Figure 2. Lifetimes of the 15 bp (green), 30 bp (red), and 45 bp (black) NP—dsDNA —dye assemblies for FAM (a) and for Cy5 (b) relative to dsDNA —dye controls (blue, top). The data have been normalized and offset vertically for viewing. Single-exponential fits through the data are shown (—).

Figure 4. (a) Absorption and corrected spectra for the FAM—dsDNA— NP system and (b) absorption spectra and corrections for Cys—dsDNA — NP. These spectra compare purified dye—dsDNA—NP (black, offset 0.05 au—FAM: 0.10 au—Cy5), dye—-dsDNA in buffer (- - -), and the background subtracted absorption spectrum of the dye—dsDNA—NP to correct for NP absorption (—).

Figure 4. (a) Absorption and corrected spectra for the FAM—dsDNA— NP system and (b) absorption spectra and corrections for Cys—dsDNA — NP. These spectra compare purified dye—dsDNA—NP (black, offset 0.05 au—FAM: 0.10 au—Cy5), dye—-dsDNA in buffer (- - -), and the background subtracted absorption spectrum of the dye—dsDNA—NP to correct for NP absorption (—).

Table 1. Measured Values for the Quantum Yield of Quenching Efficiency for the Three Strands of dsDNA Based on cw-PL Spectra (PL Qes) and on Lifetime Quenching (tQe#)

Table 1. Measured Values for the Quantum Yield of Quenching Efficiency for the Three Strands of dsDNA Based on cw-PL Spectra (PL Qes) and on Lifetime Quenching (tQe#)

Figure 5. (a) Graphic representation of a donor dye—nanometal acceptor pair separated by dsDNA approximated as a rigid rod of length, d. The donor is treated as a localized dipole, and the acceptor is assumed to have overlap at all stearadians. (b) Pictorial representation of a gold NP in an idealized electric field of a nearby molecular dipole. All surface dipole scattering events associated with the free electrons of the gold are shown perpendicular to the surface, which are predicted to be the dominant contributors to the NSET process. 4 The calculated and theoretical rates of energy transfer to the metal surface are calculated using eqs 3 and 7, respectively.

Figure 5. (a) Graphic representation of a donor dye—nanometal acceptor pair separated by dsDNA approximated as a rigid rod of length, d. The donor is treated as a localized dipole, and the acceptor is assumed to have overlap at all stearadians. (b) Pictorial representation of a gold NP in an idealized electric field of a nearby molecular dipole. All surface dipole scattering events associated with the free electrons of the gold are shown perpendicular to the surface, which are predicted to be the dominant contributors to the NSET process. 4 The calculated and theoretical rates of energy transfer to the metal surface are calculated using eqs 3 and 7, respectively.

Figure 6. Quenching data for FAM (a) and Cy5 (b) based upon photoluminescence (a) and lifetimes (lM) overlaid on top of a theoretical curve generated using eq 9.

Figure 6. Quenching data for FAM (a) and Cy5 (b) based upon photoluminescence (a) and lifetimes (lM) overlaid on top of a theoretical curve generated using eq 9.

Figure 7. Absorption comparison of the dye—15mer (w/o NP, - - -) to the dye—15mer—NP absorption, after subtracting gold absorption and correcting for scattering (—). All absorbances have been normalized at the DNA absorption wavelength, 260 nm. A small scattering correction has been applied to the NP—15mer—FAM absorption to correct the baseline from the 200—450 nm range. Absorption comparison for both FAM—15mer— NP (a) and CyS—15mer—NP (b) allows direct monitoring of the oscillator strength for the dye at the closest proximity to the gold NP measured here.

Figure 7. Absorption comparison of the dye—15mer (w/o NP, - - -) to the dye—15mer—NP absorption, after subtracting gold absorption and correcting for scattering (—). All absorbances have been normalized at the DNA absorption wavelength, 260 nm. A small scattering correction has been applied to the NP—15mer—FAM absorption to correct the baseline from the 200—450 nm range. Absorption comparison for both FAM—15mer— NP (a) and CyS—15mer—NP (b) allows direct monitoring of the oscillator strength for the dye at the closest proximity to the gold NP measured here.

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