The Effect of an Active Substrate on Nanoparticle‐Enhanced Fluorescence

Abstract

Strong resonant coupling between light and plasmons in noble metal particles of nanometer dimensions leads to a number of striking and technologically important optical effects, among them surface enhanced Raman scattering (SERS) [1] and the enhancement of fluorescence from nearby molecules. [2] While each of these show great promise for the development of highly sensitive biochip detectors, [3,4] fluorescence is the technique of choice for many biological assays. Significant enhancement here would greatly enhance the sensitivity of these assays to a host of target biomolecules. To date, the maximum enhancement available in fluorescence has not been established. This is largely due both to difficulties in controlling the size and shape of the particles, and to the multiplicity of contributing factors: increased radiative decay rate and enhanced electric fields at resonance, ''hot spots'', i.e. regions of high field between closely spaced particles. The substrate is known to play a role as well; in particular there have been suggestions that certain substrates might play an active role in light-plasmon coupling rather than merely shifting the resonance frequency, as would a conventional glass substrate. Here we report observations of just such an effect, in which the size dependence of the enhancement of fluorescence from monodisperse silver nanoparticles is profoundly altered by the use of a Si substrate. Comparing fluorescence measurements with calculations of the response of the silver nanoparticles to incident light, we show that unlike what is commonly assumed, the variation of the fluorescence enhancement with nanoparticle diameter does not simply follow that of plasmon excitation as measured by the optical extinction. Instead we find that it is the generation of regions of high electrical field intensity near the particle which dominates the fluorescence enhancement we observe, and that the silicon substrate plays an active role in this regard: sweeping these regions out from beneath the particles as their size approaches the optimum for fluorescence. ResultsIn Figure 1 we compare scanning electron microscopy images and fluorescence microscopy images from size-selected, fluorescently-tagged silver nanoparticles on a silicon substrate over a range of particle diameters. In Figure 1(a)-(e) the SEM images are shown in red, and fluorescence images for an excitation wavelength of 514 nm (exciting the Cy3 fluorophore), scanned from precisely the same regions are superposed in green. The SEM images show that particles are spherical, and randomly distributed. There are rare occasions where two or more particles are in contact; this occurs more frequently on samples with larger average particle diameter, as shown in Figure 1(e). In general, however, individual particles are well separated from their neighbors. The fluorescence intensity shows a strong and systematic variation from image to image, i.e. as the average diameter of the Ag nanoparticles is varied. Highest intensities are obtained from samples with partic...

Effect of Extended Extinction from Gold Nanopillar Arrays on the Absorbance Spectrum of a Bulk Heterojunction Organic Solar Cell

“…For the array with W = 180 nm × P = 360 nm, the most intense peak in extinction occurs at a wavelength of 680 ± 4 nm (blue curve). This red shift in the peak in extinction as the NP width increases is consistent with the known behavior of localized surface plasmon resonances [14,17,18]. Figure 2b shows the experimentally derived extinction spectra for a P3HT:PCBM film deposited on a non-NP-patterned substrate (black curve), for a P3HT:PCBM film on an Au NP array with W = 95 nm × P = 200 nm (red curve), and for a P3HT:PCBM film-coated NP array with W = 180 nm × P = 360 nm (blue curve).…”

“…This however is the opposite of what we have seen in the correspondence between our extinction measurements and the EQE measurement; it leads us to ask whether field enhancement within the organic layer does occur at wavelengths corresponding to particle plasmon excitation in our devices. Fluorescence maps of molecular films on metallic nanostructures have been demonstrated to reveal the excitation of particle plasmon and surface plasmon resonances in such structures [17][18][19][20][21]; locations in the map where enhancement in fluorescence occurs indicate regions of localized field enhancement. The insert in Figure 4 shows a fluorescence map across three different P3HT:PCBM film-covered NP arrays on an ITO coated glass substrate (for this measurement no Al/TiOx layers were deposited).…”

“…The insert in Figure 4 shows a fluorescence map across three different P3HT:PCBM film-covered NP arrays on an ITO coated glass substrate (for this measurement no Al/TiOx layers were deposited). The image was measured with a scanning laser microscope using a 633 nm wavelength source, collecting fluorescence at wavelengths above 660 nm [17]. The visible square regions in the map are from three Au nanopillar arrays; the pillar width and pitch are (left square) W = 95 nm × P = 180 nm, (center square) W = 95 nm × P = 300 nm, and (right square) W = 180 nm × P = 360 nm.…”

Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

“…In addition, for nanoparticles having a broad size distribution, it is difficult to determine if the increase in particle size is due to molecular conjugation or particle aggregation. Spectroscopic methods [52][53][54] provide an orthogonal comparison to these physical characterization approaches. By monitoring the change in optical signals corresponding specifically to BSA (absorption or fluorescence), it is possible to track the change in surface density of BSA on AuNPs.…”

Nanoparticle film deposition using a simple and fast centrifuge sedimentation method

“…11c). As noted by Guo (2008), substrates are known to play an active role in the fluorescence of semiconductor nanoparticles. Metal surfaces are known to have a quenching effect on nanoparticle fluorescence signals (Ganesh et al 2008;Matsuda et al 2008).…”