Metal-enhanced fluorescence using anisotropic silver nanostructures: critical progress to date

Abstract

In this critical and timely review, the effects of anisotropic silver nanostructures on the emission intensity and photostability of a key fluorophore that is frequently used in many biological assays is examined. The silver nanostructures consist of triangular, rod-like, and fractal-like nanoparticles of silver deposited on conventional glass substrates. The close proximity to silver nanostructures results in greater intensity and photostability of the fluorophore than for fluorophores solely deposited on glass substrates. These new anisotropic silver nanostructure-coated surfaces show much more favorable effects than silver island films or silver colloid-coated substrates. Subsequently, the use of metal-enhanced fluorescence (MEF) for biosensing applications is discussed.

Fig. 1

Classical Jablonski diagram for the free space condition and the modified form in the presence of metallic particles, islands, colloids, or silver nanostructures. Eexcitation, E_mmetal-enhanced excitation rate, Γ_m radiative rate in the presence of metal. For our studies, we do not consider the effects of metals on k_nr. Adapted from [1]

Fig. 2A–B

A The effect of an increase in radiative decay rate on the lifetime and quantum yield. B Metallic surfaces can create unique fluorophores with high quantum yields and short lifetimes. Adapted from [1]

Fig. 3A–B

A Fluorophore near a metallic spheroid. B The resonant frequency of the dye is assumed to be 25,600 cm⁻¹, approximately equal to 391 nm. The volume of the spheroids is equal to that of a sphere with a radius of 200 Å. Adapted from [1]

Fig. 4

Technique for the deposition of silver nanorods and triangular nanoplates on glass substrates

Fig. 5A–B

A Absorption spectra of silver nanorods in solution and on glass deposited by slow deposition method, B absorption spectra of silver nanorods deposited on glass substrates by rapid deposition method. Adapted from [29]

Fig. 6

Absorption spectra of silver triangular nanoplates deposited on glass substrates by a rapid deposition method. Adapted from [30]

Fig. 7A–B

A AFM image of silver nanorods deposited on glass substrate, B AFM image of silver triangular nanoplates deposited on glass substrate. Adapted from [29, 30]

Fig. 8A–B

A Fluorescence emission intensity of ICG–HSA on silver nanorods with low (A₆₅₀=0.10) and high loading density (A₆₅₀=0.48). The arbitrary emission intensity of ICG–HSA on glass is 250. B Normalized emission intensities of ICG–HSA on glass and on silver nanorods (A₆₅₀=0.48), which show identical spectral behavior. Adapted from [29]

Fig. 9A–B

A Fluorescence emission intensity of ICG–HSA on silver triangles with different loading densities. The emission intensity of ICG–HSA on glass is 75. B Normalized emission intensities of ICG–HSA on glass and on silver nanorods (A₅₅₀=0.03), which show identical spectral behavior. Adapted from [30]

Fig. 10A–B

A Photostability of ICG–HSA on silver nanorods prepared by the rapid deposition method, and B with the laser power adjusted to yield the same initial steady-state intensity. Adapted from [29]

Fig. 11A–B

A Photostability of ICG–HSA on silver triangular nanoplates prepared by the rapid deposition method, and B with the laser power adjusted to yield the same initial steady-state intensity. Adapted from [30]

Fig. 12

Complex intensity decays of ICG–HSA in buffer (B), deposited on glass slides (G), and on silver nanorods deposited by the fast deposition technique (Ag). The RF denotes the instrumental response function