NANO LETTERS
Highly Efficient Multiphoton-Absorption-Induced Luminescence from Gold Nanoparticles
2005 Vol. 5, No. 6 1139-1142
Richard A. Farrer, Francis L. Butterfield, Vincent W. Chen,† and John T. Fourkas*,‡ Eugene F. Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467 Received April 13, 2005; Revised Manuscript Received May 5, 2005
ABSTRACT We demonstrate that highly efficient photoluminescence is generated from gold nanoparticles as small as a few nanometers in diameter upon irradiation with sub-100-fs pulses of 790-nm light. Strong emission is observed at excitation intensities comparable to or less than those typically used for multiphoton imaging of fluorescently labeled biological samples. The particles have polarized emission, can radiate more efficiently than single molecules, do not exhibit significant blinking, and are photostable under hours of continuous excitation. These observations suggest that metal nanoparticles are a viable alternative to fluorophores or semiconductor nanoparticles for biological labeling and imaging.
Optical techniques for detecting single molecules are opening broad new avenues into the exploration of microscopic structure and dynamics.1,2 However, single-molecule detection techniques have yet to reach their full potential, particularly in the biological arena. Single molecules undergo permanent photobleaching on a time scale that is prohibitive for many experiments and exhibit significant blinking over shorter time scales. Photoluminescent semiconductor nanoparticles3 alleviate the first of these difficulties, but are still prone to blinking, can be difficult to anchor to molecules of interest, and are larger than most organic dye molecules.4 The use of noble-metal nanoparticles as single-molecule probes is attractive in that they are readily synthesized with controlled diameters, relatively inert, and easily attached to other moieties. A first step in this direction was taken recently by Boyer et al., who demonstrated the photothermal detection of single gold nanoparticles.5 However, the photothermal technique requires the use of laser radiation at a wavelength that is absorbed by many biological materials and also causes heating in the neighborhood of the detected particle, both of which are significant disadvantages in biological contexts. Peyser and Dickson have demonstrated efficient luminescence from clusters of a few silver atoms generated on silver oxide films.6 While it was subsequently shown possible to generate such clusters within dendrimers,7 there is no available means of attaching them to other molecules in a * Corresponding author. E-mail:
[email protected]. † Present address: School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332. ‡ Present address: Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-4454. 10.1021/nl050687r CCC: $30.25 Published on Web 05/18/2005
© 2005 American Chemical Society
controlled chemical manner. Third-harmonic generation from gold nanoparticles has been demonstrated recently as well, but only for rather large nanoparticles.8 Another promising technique for observing metal nanostructures9,10 is multiphoton-absorption-induced luminescence (MAIL). In this technique, absorption of multiple photons from a nearinfrared ultrafast laser can lead to efficient luminescence of a noble-metal nanostructure. To date, the smallest reported object on which MAIL has been implemented is a 100 nm gold nanoparticle.10 Here we demonstrate that highly efficient MAIL can be generated from gold nanoparticles over a broad range of dimensions at laser intensities that are at or below those generally used for two-photon imaging of living cells. Our experimental apparatus is similar to one reported previously.11 The excitation source was a home-built Ti:sapphire laser that produces sub-100-fs pulses at a repetition rate of 76 MHz with a center wavelength of 790 nm. The laser was directed into the back port of an inverted microscope using galvanometric scanning mirrors and reflected into the back aperture of a 1.3 NA, 40× objective using a dichroic mirror. Luminescence was collected through the same objective, passed through the dichroic mirror, split into orthogonal polarizations, and then collected by single-photon-counting avalanche photodiodes. Emission spectra were obtained by placing a fiber-optic spectrometer at the camera port of the microscope. Single-photon excitation of the interband transitions of gold nanoparticles generally leads only to weak luminescence, except in the case of particles with diameters of 5 nm or less.12 However, in the case of multiphoton excitation
Figure 1. False-color emission images for (a) 125-nm, (b) 60-nm, (c) 15-nm, and (d) 2.5-nm gold nanoparticles. The images are approximately 15 µm on a side. A representative real-color MAIL image and contour intensity plot of 15-nm silica-coated Au particles on a microscope cover slip is shown in (e).
the photoluminescence efficiency can change dramatically due to field enhancement effects.9 The existence of asymmetries in noble metal structures can lead to electric field in enhancements of many orders of magnitude, leading to high multiphoton absorption cross sections. The degree of this asymmetry is of prime importance in determining the efficiency of MAIL. Highly symmetric gold nanoparticles comprising 11 or 55 atoms exhibit only weak photoluminescence upon multiphoton excitation. However, we have found that less regularly shaped gold nanoparticles exhibit efficient MAIL over a broad range of diameters. Figures 1a-d are false-color MAIL images of gold nanoparticles ranging in diameter from as large as 125 nm to as small as 2.5 nm. The nanoparticles were spin coated from dilute solution onto a microscope cover slip, and an excitation power of approximately 5 mW was employed. The broad range of intensities observed is indicative of the dependence of the degree of field enhancement on the shape of each particle. A similar range of intensities is observed when more dilute suspensions are employed, which serves as evidence that particle aggregation is not responsible for the efficient emission. There are differences in the MAIL efficiencies of particles with different diameters, although it is difficult to quantify these differences precisely due to the range of efficiencies present for particles of the same average diameter. On average, the 60-nm particles exhibited the strongest MAIL and emission from 2.5-nm particles was weaker on average by a factor of approximately 3 to 4. Considering the fact that the probability for multiphoton absorption is proportional to the field enhancement to the power of six (vide infra), the differences in MAIL efficiencies for different particle sizes are quite modest. These differences arise from a complex interplay of a number of factors. First, as the particles get smaller, the increased degree of confinement leads to changes in the density of interband states. In addition, in small particles the momentum conservation requirement for radiative electron-hole recombination is relaxed to some extent.13 Both of these effects would tend to favor stronger 1140
emission from smaller particles. On the other hand, larger gold particles exhibit greater electric field enhancements in the near-infrared.14 Also, the larger particles employed here do tend to be more asymmetric than the smaller ones, which also promotes increased field enhancement.14 Field enhancement is well-known to be strong at the junction of two or more particles.15 To rule out any contribution of aggregation to the MAIL efficiency, we prepared 15-nm gold nanoparticles and coated them with a layer of silica.16 A dilute solution of the particles was spincoated on a cover slip and imaged. The dwell time was
