Up-Conversion Luminescence of Gold Nanospheres When Excited at

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Up-Conversion Luminescence of Gold Nanospheres When Excited at Nonsurface Plasmon Resonance Wavelength by a Continuous Wave Laser Bhanu Neupane, Luyang Zhao, and Gufeng Wang* Chemistry Department, North Carolina State University, Raleigh, North Carolina 27695, United States S Supporting Information *

ABSTRACT: We show that, when gold nanospheres are excited at the red side of the surface plasmon resonance (SPR) wavelength at 592 nm by a continuous wave (CW) laser, they give substantial up-converted luminescence in the SPR wavelength range. The luminescence intensity scales as a second-order function of the excitation power, with a quantum yield ∼1/50 of down-conversion luminescence when illuminated at a power of 30 MW/cm2. The luminescence spectrum is completely different than the SPR profile, indicating a new emission mechanism possibly involving interband transitions coupled with phonons or localized vibration of neighboring gold atoms. Such luminescence is also observed to be substantial for short gold nanorods with an aspect ratio of ∼2 but weak for bulk gold. This study provides new insight to the understanding of gold nanoparticle luminescence and opens a new detection scheme for gold nanoparticle-based biological imaging. KEYWORDS: Up conversion, photoluminescence, gold nanospheres, gold nanorods, surface plasmon resonance (SPR), interband transition

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The luminescence can be generated by using one-photon or two-photon excitations. One-photon luminescence (1PL) of AuNP has been studied in more detail,6,8,11,12 and several applications, for example, luminescence autocorrelation spectroscopy,6 are emerging in recent years. For example, Tcherniak et al. studied the polarization dependence of 1PL of spheres and rods. They found that, for gold nanorods, the intense red spectral component is polarized parallel to the longitudinal SPR (LSPR), which strongly supports that fast interconversion between the surface plasmon and the hot electron−hole pairs occurs and the luminescence comes directly from the emission of a surface plasmon. However, the blue spectral component near transverse SPR (TSPR) wavelength is weakly polarized, which suggests that it may have mixed contributions from both the TSPR emission and the interband transitions. The above observation was confirmed by Yorulmaz et al.12 In addition, they also systematically studied on the quantum yield of gold nanorods with different aspect ratios and the difference between luminescence and SPR scattering spectra. They found that, for short rods, the luminescence is blue-shifted while for the long rods, it is red-shifted. Although the origin of the shifts is unclear, they suggested that the shift may be contributed from non-SPR emissions, that is, interband and/or intraband luminescence reminiscent of those from the bulk gold.

hotoluminescence from bulk gold, which covers from UV to IR, was reported a long time ago.1 The luminescence is weak because of efficient nonradiative decays. The origin of the luminescence is attributed to interband, that is, recombination of d-band holes with electrons in the sp band, and intraband transitions, that is, electron−hole recombination in the sp band. Later, it was reported that, in gold nanostructures,2−4 the luminescence is enhanced and cannot be accounted for just considering inter- and intraband transitions. It was concluded that the luminescence must involve the surface plasmon resonances (SPR), which is the collective oscillation of valence electrons excited by light when the size of the metallic nanostructures becomes comparable to the wavelength of the light. Currently, there are two mechanisms proposed in the literature regarding how SPR is involved in gold nanoparticle (AuNP) luminescence. In the first mechanism, excitation energy from hot electron−hole pairs is quickly transferred to the plasmon, and the radiative decay of the excited plasmon yields luminescence. This mechanism can be loosely viewed as a fluorescence resonance energy transfer (FRET) process.5−8 The second mechanism is based on the antenna effect of surface plasmons. Here, the plasmon does not emit directly but enhances the radiative recombination rate of electron−hole pair by a local surface plasmon field.3,4,9,10 As compared to that from the bulk, the luminescence is amplified and shaped to have a profile resembling closely to the SPR spectrum. It is not clear so far which mechanism dominates. © XXXX American Chemical Society

Received: April 25, 2013 Revised: July 20, 2013

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communications). Both beams were circularly polarized by quarter-wave plates (ACWP-400-700-06-4, CVI/Melles Griot) and collimated to overfill the back aperture of the microscope objective. The luminescence signals were filtered using proper combinations of dichroic mirrors and bandpass/short-pass/ long-pass filters. The detection was from either a microspectroscopic system (detection module DM1) or a confocal type imaging system (DM2). DM1 was composed of a transmission grating (70 grooves/mm, Edmund) and a CMOS camera (Hamamatsu ORCA 2.8). The spectrometer was calibrated using three different laser lines, and the spectral resolution was estimated to be ∼5 nm. DM2 was a confocal type imaging system using the combination of a 50 μm, multimode fiberoptics (M16L01, Thorlabs, NJ) serving as the confocal pinhole and an avalanche photodiode (SPCM-AQRH15-FC, Perkin-Elmer) serving as the detector. Both excitation laser lines were aligned to overlap in all xyz directions so that the same particles can be excited simultaneously/alternatively. The confocal image was acquired by scanning the sample using a piezo-stage (PI Nano, Physik Instrumente) mounted on a manual XY translational stage. The precision of the piezo-stage was 1.0 nm. The luminescence spectrum of 60 nm spheres excited at 488 nm, that is, the higher energy side of the SPR band, and collected at λ > 510 nm (excitation/emission scheme 1a in Figure 2) is shown in Figure 3A (black curve). This spectrum

Photoluminescence of AuNP can also be observed when using two-photon excitation. The inherent spatial confinement of two-photon luminescence (2PL) along with the brightness and stability offered by gold nanoparticles make it ideal for three-dimensional (3D) imaging and tracking studies.13,14 However, 2PL properties of gold nanoparticles are more complicated, and so far inconsistent spectra were reported. For example, Imura et al.10 and Bouhelier et al.7 independently reported that the 2PL spectra of individual gold nanorods closely resemble corresponding SPR scattering peaks. On the other hand, distinctively different luminescence spectra from individual nanorods have been reported by Wang et al.14 and Beversluis et al.4 The intraband transitions may contribute to the difference in the observed 2PL spectra in the red to IR regions.4 The great majority of the above studies focused on the luminescence generated by exciting the nanoparticles at a wavelength on or blue-shifted from the SPR wavelengths using either one-photon or two-photon excitation. In our recent work of building a continuous-wave stimulated emission depletion (CW-STED) microscope that uses 488 nm excitation and 592 nm depletion,15 surprisingly, we observed fairly intense, upconverted luminescence from gold nanospheres and short nanorods when they were illuminated by the CW 592 nm laser. These gold nanoparticles were clearly visible in the microscope eyepiece when illuminated by the 592 nm laser, giving a green image with a different shade than the normal down converted luminescence. In this manuscript, we study the up conversion luminescence of gold nanoparticles by selectively exciting at 592 nm, the red side of the SPR of gold nanospheres. Single particle spectra and images were acquired using our home-built microspectroscopic and imaging system in Figure 1. It was modified from our confocal-type STED microscope by removing the vortex phase plate in the depletion beam path.15 The excitation can be toggled between excitation module 2 (EM2, 488 nm excitation from an Ar ion laser, 35-LAP-431240, CVI/Melles Griot) and excitation module 1 (EM1, 592 nm excitation from a fiber laser, VFLP-1000-592-OEM1, MPB

Figure 2. Excitation and detection schemes used in collecting single particle photoluminescence spectra. For comparison, their corresponding extinction spectra in the bulk solution are also shown. Black, blue, and red spectra are for spheres, short rods, and long rods, respectively.

resembles closely to the single particle scattering spectra (data not shown), which is consistent with the literature.4,6,12 Note that the interband emission from bulk gold has a very similar spectrum when excited at 488 nm (black curve in Figure S1 in the Supporting Information).4,16−18 The strong luminescence from gold nanospheres may come from the SPR emission and/ or the interband transition amplified by SPR as suggested previously.4,6 When we excited the same spheres at 592 nm, that is, the lower energy side of the SPR band, interestingly, we observed a substantial up-converted luminescence in the same emission range (510−560 nm, scheme 2b). More importantly, the upconverted emission (black-dotted curve in Figure 3B) does not resemble the gold nanosphere SPR peak. For clarity, the

Figure 1. Schematic of the confocal microscopy and spectroscopy system. EM: excitation module; DM: detection module; L: lenses; QWP: quarterwave plate; DCSP: dichroic mirror (short pass); DCLP: dichroic mirror (long pass); NF: notch filter; BP: bandpass filter; LP: long-pass filter; APD: avalanche photodiode detector. B

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in Figure 3C) intensity decreases quickly as a function of the wavelength, giving a profile matching the SPR band. We also measured the luminescence properties of long gold nanorods (25 × 92 nm). At 592 nm, the absorption crosssection of long rods is small (Figure 2). The emission spectrum above 655 nm of long rods when excited at 592 nm (solid red curve in Figure 3D) is practically identical to that when excited at 488 nm (red-dotted curve in Figure 3D). We also observed up-converted luminescence from long rods (Figure S2 in the Supporting Information) at the wavelength 655 nm, scheme 2c). The luminescence (black-solid curve in Figure 3C) is strong, similar to that when excited at 488 nm. Interestingly, the luminescence shows an extended, structureless emission peak. Excitation at this wavelength can have mixed contributions from both the tail of the SPR and the inter/intraband transitions. We then measured the luminescence of short gold nanorods (25 × 51 nm). When excited at the blue side of the TSPR (blue curve in Figure 3A), the short rods show strong luminescence with a profile similar to their scattering spectrum. They also show substantial up-converted luminescence (quantum yield ∼2 × 10−8) when illuminated at 592 nm (solid blue curve in Figure 3B), which could excite both the interband transitions and the LSPR. Like the spheres, the shape of this spectrum is very different from the 1P down conversion luminescence (blue-dotted curve in Figure 3B). When excited at 592 nm and collected beyond 660 nm, the strong luminescence (blue curve C

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This model can gain support from excitation powerdependent luminescence intensity and spectral shift. Figure 5A shows the scaling of luminescence intensity as a function of

Figure 4. Luminescence spectra of a sphere (frame A) and a short rod (frame B) with CW and pulsed modes excitation at 592 nm. In both frames, blue and black curves are for the pulsed and CW excitations, respectively. The red curve represents the difference between the CW and pulsed excitation spectra. The focal plane pulsed and CW laser intensities used were 12 MW/cm2 and 1 GW/cm2, respectively.

luminescence. This further confirms that the up-conversion luminescence obtained under CW excitation is not due to twophoton process. To find out the origin of this mystery of luminescence, we also measured the up-converted emission from several other gold samples including bulk gold (250 nm gold film), scratches on bulk gold, and ultralong rods with an aspect ratio of ∼10 (25 × 250 nm). We found that all gold samples have this upconverted luminescence. However, the luminescence is weak for bulk gold and is enhanced in nanostructures, with gold spheres and short rods having the largest enhancement. For example, bulk gold (250 nm thick template-stripped gold film with atomic level flatness) also shows this up-converted emission with a similar spectral shape (Figure S2 in the Supporting Information), but its quantum yield is ∼100 times smaller than that of nanospheres or short nanorods. Interestingly, the great enhancement of luminescence on artificial scratches can be directly visualized in the microscope eyepiece as the laser beam scans across those scratches. The luminescence profile of the scratches also has a similar profile (Figure S2), with an enhancement factor of several tens to ∼100 times over bulk gold depending on their locations. Similarly, the ultralong gold rods (25 × 250 nm) show a similar emission spectrum (Figure S2 in the Supporting Information), whose brightness is ∼10 times higher than bulk gold, or on the same level of the long rods (25 × 51 nm). Our homemade cetrimonium bromide (CTAB)-capped gold nanospheres also have a luminescence spectrum similar to those from other samples (see supporting figure S2). These observations confirm that the luminescence is prevalent in all gold samples. The luminescence is enhanced in nanostructures, with the greatest enhancement observed in short rods and spheres. Since the luminescence is up-converted, it must acquire additional energy from some source(s). The luminescence spectral intensity decreases as the gap between the excitation and emission increases, possibly suggesting that the emission is originated from the interband transitions (radiative recombination of electron−hole pairs) shaped by the coupling with phonons or local atomic vibrations; that is, the additional energy in higher energy interband transition is obtained from the vibration energy of the gold lattice (phonons)17 or neighboring gold atoms (localized vibration). Such a mechanism gives a scenario that the excitation of one interband transition causes an “overspill” to neighboring interband transitions with higher energies through vibrational relaxation.

Figure 5. (A) Photoluminescence intensity as a function of the laser power for short nanorods (25 × 51 nm). Nonlinear least-squares fittings (second order) is shown in red. (B) Photoluminescence spectra obtained for short rod with different CW laser intensities. The intensity for red, blue, and black curves is 40, 28, and 12 MW/cm2 respectively. (C) Photostability of the short rods excited by 488 and 592 nm, respectively. Laser power at the focal plane for 488 and 592 nm laser was 2 MW/cm2 and 30 MW/cm2, respectively. (D) Photoluminescence spectra of a short gold nanorod before and after 592 nm laser illumination.

the CW laser power, which displays a second-order nonlinearity as disclosed by polynomial fitting. Note that the observed nonlinearity is not, as proved earlier, due to a two-photon process. In the phonon coupling model (Supporting Information), the laser excitation heats the particle,6,18,19 which enhances the coupling between the phonons and interband transitions and results in a much higher luminescence intensity. Data shown in Figure 5A can be satisfactorily fitted with this model as shown in Figure S4 in the Supporting Information. The fitting, although not directly proves, shows that it is possible that the phonon−interband transition coupling can be the origin of observed up-conversion luminescence. More importantly, we observed a blue shift in the luminescence spectrum when the excitation laser is increased from 12 MW/cm2 to 40 MW/cm2 (Figure 5B). This shift can be qualitatively explained using the phononcoupling model: the increase of the laser power will increase the local temperature, which activates higher frequency phonons or localized vibrations. The probability of occupation of higher energy electron−hole states increases due to phononic or vibrational coupling, resulting in a blue shift of the emission.20−22 It is unclear why the phonon-assisted up converted luminescence is greatly enhanced in nanostructures (a factor of ∼100 times increase in quantum yield). One possible mechanism is that the enhancement is purely based on the metallic process; that is, heating increases the coupling between interband transition and phonons/localized vibrations. As D

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nanorods opens up new possibilities in various imaging applications. To summarize, our study shows a new type of up-conversion luminescence for gold nanospheres and nanorods (25 × 51 nm) when excited at non-SPR (or non-TSPR) wavelengths. The luminescence intensity scales nonlinearly as a function of the excitation power (second order), with a quantum yield ∼1/ 50 of down-conversion luminescence when illuminated at a moderate laser power (30 MW/cm2). The luminescence profile is very different from the typical down-conversion luminescence and does not follow the SPR band profile. These suggest that the up-conversion luminescence may not originate from the SPR decay. One possible pathway is that the interband transitions couple with photons or local vibrations during which additional energy is acquired. This study helps us understand the fundamentals of gold nanoparticle luminescence. In addition, we show that the up-converted luminescence has a relatively high yield and is photostable. It provides a different excitation scheme and can be used for the purpose of imaging and potentially in many other applications.

compared to long rods and bulk gold, short rods and spheres have relatively larger absorption cross sections at 592 nm. Laser induced heating scales linearly with absorbance (equation S1 in Supporting Information), which increases the coupling strength and also the luminescence intensity (equation S2 in Supporting Information). Thus, the stronger luminescence observed in short rods and spheres is due to more effective local heating of the nanoparticles. The phonon density of states available for coupling may also contributes to the difference for different gold samples. However, we cannot exclude the possibility that the up-conversion process is enhanced via antenna or excitonic effect by SPR; that is, SPR enhances the radiative recombination rate of electron−hole pairs by a local surface plasmon field. To have a comprehensive understanding of this up-converted photoluminescence, we also tested the photostability of the luminescence signal and compared it with the down-converted luminescence for the same particles. Although a small fluctuation in signals can be identified, possibly due to thermal fluctuations, the up-conversion signal is fairly stable over the time window of more than 2 min (Figure 5C). We measured the luminescence spectra for the same particle before and after the laser irradiation (Figure 5D). Spectra shown in Figure 5D are practically identical, suggesting that the luminescence is not caused by melting or deformation of the gold nanoparticles. This is also consistent with the interband−phonon coupling model, which predicts a maximum temperature caused by laser heating well below the melting temperature of gold nanoparticles (in the range of 1000−1200 K).23−25 Finally, we explored the imaging application of this upconverted luminescence. Confocal images of the immobilized short rods were obtained by scanning the sample in the XY plane for the two laser excitations, respectively. Figure 6A−D shows that the nanospheres and nanorods can be imaged with both 488 and 592 nm laser excitations. A decent signal-to-noise ratio (>50) can be obtained at a pixel integration time of 1 ms. This up-converted luminescence of gold nanospheres and

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ASSOCIATED CONTENT

S Supporting Information *

Materials and methods, a theory section, and five supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the North Carolina State University start-up funds to G.W. We acknowledge the Analytical Instrumentation Facility (AIF) center, North Carolina State University, for providing SEM measurements. We acknowledge Professor Paul Maggard from Chemistry Department and Professor Joseph Tracy from Materials Science and Engineering Department, NCSU, for providing equipment in this study. Nanopartz Inc. is acknowledged for providing a variety of gold nanoparticles for testing.

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REFERENCES

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Figure 6. Confocal image of gold particles immobilized on glass slide. Images A and B are for spheres; images C and D are for short rods. The pixel integration time was 1 ms. Scale bar 2 μm. E

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