J. Phys. Chem. C 2009, 113, 6003–6008
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Electrochemically Controlled Surface Plasmon Enhanced Fluorescence Response of Surface Immobilized CdZnSe Quantum Dots Petra J. Cameron,*,† Xinhua Zhong,‡ and Wolfgang Knoll§ Department of Chemistry, UniVersity of Bath, Bath BA2 7AY, United Kingdom, Department of Chemistry, East China UniVersity of Science and Technology, 200237 Shanghai, China, and Austrian Research Centres, Tech Gate Vienna, Donau-City-Strasse 1, 1220 Wien, Austria ReceiVed: December 4, 2008; ReVised Manuscript ReceiVed: February 3, 2009
Cd0.1Zn0.9Se quantum dots with fluorescent emission centered on 614 nm were covalently coupled to a 11amino-1-undecanethiol monolayer self-assembled on a gold surface. A 594 nm laser was used to excite surface plasmons in the gold film and the resulting surface plasmon enhanced fluorescence of the quantum dots was measured with a photomultiplier. The application of negative potentials (versus Ag/AgCl) led to a decrease in the surface plasmon enhanced fluorescence signal, the fluorescence signal recovered if the cell potential was returned to 0 V or open circuit. These results show that the fluorescence emission of attached quantum dots can be tuned by the application of an electrical potential in an aqueous environment, which may be relevant to quantum dot applications in biosensing. Introduction Quantum dots are increasingly being used as fluorescence markers for biosensing, often in sensing systems where the binding of a labeled molecule at a conducting surface is followed colorimetrically or by an increase in surface enhanced fluorescence due to interactions between the QD and surface plasmons in a metallic layer.1,2 This leads to the interesting question of how the properties of quantum dot labeled molecules change if an electrochemical potential is applied to the surface that they are attached to. Changes in QD fluorescence with the application of an electrochemical potential have been demonstrated for thin films of QDs in anhydrous organic solvents,3-8 but to have relevance to biosensing the experiments must be carried out in an aqueous environment. In this paper electrochemically induced fluorescence changes in QDs are investigated in an aqueous environment and in the presence of oxygen. Photoluminescence (PL) enhancement in quantum dots when placed in proximity to surface plasmon fields has been shown on silver, aluminum, and gold surfaces. Photoluminescence is only seen if the absorption band edge for the quantum dots (QDs) occurs at a longer wavelength than the surface plasmon excitation wavelength.9-13 In the case of quantum dots close to (few tens of nanometers away from) gold surfaces, the photoluminescence is almost entirely quenched if the gold surface is atomically flat. If the gold is rough, either due to deliberate nanostructuring or as a function of the deposition method, then the quantum dots show photoluminescence enhancement. Okamoto et al.9 measured a 23-fold enhancement in plasmon induced photoluminescence from CdSe quantum dots directly deposited on evaporated gold films (as compared to CdSe on quartz). The as-deposited surface roughness was of the order of a few tens of nanometers and was sufficient to induce substantial PL enhancement without complex nanostructuring of the gold surface. Ito et al.10 have measured the photolumi* To whom correspondence should be addressed. E-mail: p.j.cameron@ bath.ac.uk. † University of Bath. ‡ East China University of Science and Technology. § Austrian Research Centres.
nescence of CdSe/ZnS core-shell quantum dots when placed on structured gold surfaces. The rough gold was formed by Ar+ etching of gold films and had surface structure in the range of 20-50 nm. In their case they measured an order of magnitude increase in photoluminescence compared to the same nanoparticles on quartz glass, and the increase was attributed to the interaction of the quantum dot with the electric field of localized plasmons in the gold nanostructures. The authors also found that the photoluminescent blinking of single quantum dots was suppressed on both flat and rough gold when compared to on quartz glass. Eychmueller et al.11 deposited quantum dots onto gold nanoparticle films. A polyelectrolyte spacer layer was used to investigate the influence of the gold-quantum dot distance. A 10-fold enhancement of quantum dot photoluminescence was measured for dots emitting at 667 nm (the gold nanoparticles were excited at 480 nm), and the optimum gold-quantum dot spacing was ∼7.6 nm. Song et al. and Gryczynski et al.12,13 have also measured surface plasmon enhanced fluorescence of quantum dots on periodic gold nanoparticle arrays and silver layers, respectively. Separate experiments have shown that the photoluminescence intensity of quantum dots arrayed on gold, platinum, or indium tin oxide (ITO) can be controlled electrochemically. It has been shown that the injection of electrons into empty states in the q-dots by the application of a negative potential difference leads to marked changes in their absorption and PL properties.3-8 The fluorescence of QDs and quantum rods can also be reversibly switched by the application of a large external electric field. Vanmaelkberg et al.3 have shown that electrons can be controllably injected from ITO and gold electrodes into CdSe nanoparticles with diameters between 2.9 and 8.4 nm. They concluded that electrons are first injected into localized band gap states (at ca. -0.45 V versus NHE) before being injected into conduction band states (below -0.76 V versus NHE). The authors concluded that spectator electrons in the band gap led to a red shift of transition energies in the absorption spectrum; additional electrons in conduction band states led to a quenching of transitions in the absorption spectrum. Wang et al.4 deposited a film of ∼6.8 nm CdSe QDs with TOPO caps on ITO and
10.1021/jp8106765 CCC: $40.75 2009 American Chemical Society Published on Web 03/23/2009
6004 J. Phys. Chem. C, Vol. 113, No. 15, 2009 measured them in anhydrous N,N-dimethylformamide with 0.1 M tetrabutylammonium tetrafluoroborate background electrolyte. Changes in the photoluminescence intensity were measured as electrons were injected into and removed from the QDs electrochemically under negative bias. Charging the 1Se state led to changes in the visible absorption spectrum with an absorption bleach due to the presence of the injected electrons. It was found that there was a clear difference in kinetics between charging/discharging the 1Se state and fluorescence quenching; electron injection into the CdSe nanoparticles led to PL quenching on a slower time scale than the optical bleach. The PL intensity recovered once the negative bias was removed; the authors concluded that the PL quenching was partly due to the buildup of nonradiative recombination surface states created by injection of electrons into band gap states of the quantum dots.4-6 Both of the studies outlined above, and in fact the majority of studies of electrochemical quenching of QD PL, have been done in anhydrous organic solvents. The presence of water or oxygen has been found to increase the photoluminescence intensity of CdSe assemblies, possibly because with time the water/oxygen passivates nonradiative recombination sites on the surface of the nanoparticles.7,8 In this paper the assembly of highly fluorescent CdZnSe alloyed quantum dots on rough gold films is described. The binding of the quantum dots to the gold surface was followed by surface plasmon resonance spectroscopy; the surface plasmon field was used to excite fluorescence from the quantum dots, which was monitored perpendicular to the gold surface by using a photomultiplier tube. The gold film acted as the working electrode in a three-electrode setup, with a platinum wire counter electrode and a silver/silver chloride reference electrode completing the cell. The surface plasmon enhanced fluorescence spectra (SPFS) of the surface bound quantum dots was monitored as a function of applied electrochemical potential. It was found that the SPFS of the quantum dots was highly potential dependent. The reduction in fluorescence with negative applied potentials was reversible, with full recovery of the fluorescence signal when the cell was returned to 0 V versus Ag/AgCl or open circuit. Experimental Section Materials. 11-Amino-1-undecanethiol (Dojindo), 1-ethyl-3(3-dimethylaminopropyl)carbodiimide (EDC, Sigma Aldrich), N-2-hydroxyehtylpiperazine-N′-2-ethanesulfonic acid (HEPES, Fluka Biochemica, 99.5%), sodium sulfite, sodium hydroxide, and ethanolamine hydrochloride (Acros organics, >99%) were all used as received. Water was purified with a Millipore system and had a minimum resistivity of 18.2 MΩ. The synthesis of the CdxZn1-xSe alloyed quantum dots has been described in detail elsewhere,14 and the QDs used for this research had a Cd:Zn ratio of 1:9. The as-synthesized trioctylphosphine oxide (TOPO) capping ligands were replaced with mercaptoundecanoic acid (MUA) ligands by ligand exchange; the TOPO coated quantum dots were suspended in chloroform and then added to an aqueous solution containing a high concentration of MUA. As MUA replaced TOPO on the surface of the Cd0.1Zn0.9Se quantum dots they passed from the organic layer to the aqueous layer. The water-soluble QDs were then used for further experiments. UV-Visible Spectroscopy and Fluorescence Spectroscopy. UV-vis spectroscopy was carried out on a Perkin-Elmer Lambda 9 UV/vis/NIR spectrophotometer and fluorescence spectroscopy on a J&M TIDAS fluorescence spectrometer with illumination from a 100 W xenon bulb.
Cameron et al. Electrochemistry. Electrochemical studies were carried out with an Autolab PGSTAT 30. All electrochemistry was carried out in a 0.1 M solution of Na2SO3 adjusted to pH 12 with sodium hydroxide to prevent photodissolution of the quantum dots. The working electrode was a gold-coated LaSFN9 glass slide (Schott) and the active area was 0.5 cm2. The counter electrode was a coil of platinum wire and the reference was a silver/ silver chloride electrode (Dri-ref). Illumination of the quantum dot film was carried out through the quartz window of the flow cell, using illumination from a high-intensity blue LED (Lumiled, 470nm). Surface Plasmon Enhanced Fluorescence Spectroscopy. Surface plasmon enhanced fluorescence spectroscopy (SPR) was measured in the Kretschmann attenuated total internal reflection configuration on a home-built setup. Laser (Uniphase, HeNe λ ) 594.6 nm) light was passed through a chopper and two polarizers before being incident on one face of a LaSFN9 prism (Schott Glass). The first polarizer adjusted the intensity of the light and the second polarizer allowed only p-polarized light to reach the sample. The chopper modulated the light at 431 Hz and provided a reference signal for the lock-in amplifier. The CdZnSe quantum dot film was formed on top of a ∼45 nm gold film deposited on a LaSFN9 glass slide (Schott Glass) that was separated from the back of the LaSFN9 prism by a thin layer of index matching fluid (Cargille Laboratories Inc., n ) 1.700 ( 0.0002). The sample and the prism were mounted on a computer-operated goniometer, which was used to control the precise angle of incidence of the light. The reflected light beam was focused through a collecting lens onto a silicon photodiode. A computer program, designed in house, was used to measure the magnitude of reflected light reaching the photodiode as a function of the incident angle controlled by the goniometer. The surface plasmon enhanced fluorescence was measured by using a photomultiplier tube mounted at right angles to the gold slide, and the emitted light was passed through a collecting lens and a 611 nm (Schott DAD8 narrow band-pass filter, λm ) 611nm) filter (Schott Glass) before being incident on the photomultiplier to remove scattered laser light as far as possible. A Teflon electrochemical cell with a quartz window that could be mounted in the SPFS setup was used for all experiments. Self-Assembled Monolayers on Evaporated Gold Surfaces. Gold films were thermally evaporated onto clean LaSFN9 glass. In all cases, a 2 nm layer of chromium was evaporated just prior to gold deposition to improve adhesion of the gold layer. The evaporation was carried out with an Edwards Auto 306 evaporator; the gold films had an average thickness of 45-50 nm. Self-assembled monolayers of 11-amino-1- undecanethiol were formed by leaving the gold surface in contact with a 1 mM solution of the thiol in ethanol overnight. The SAM coverage was estimated to be about 91% from impedance spectroscopy and the SAM thickness was calculated to be 1.3 nm. The SAM coated slides were rinsed in ethanol and dried in a stream of air before being fixed in place as one wall of a Teflon flow cell, and the second wall of the flow cell consisted of a quartz window. All binding experiments and electrochemical measurements were thereafter done in situ in a combined electrochemical/SPFS flow cell. Covalent Coupling of Nanoparticles to Amine Functionalized Surfaces. Covalent coupling of quantum dots to selfassembled monolayers on gold films has been described in more detail in a previous publication.15 Briefly, the required concentration of CdZnSe nanoparticles was added to a 2 mM solution of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) in HEPES buffer at pH 7.5 to create nanoparticles with reactive
Surface Immobilized CdZnSe Quantum Dots
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SCHEME 1: Depiction of the Experimental Setup Showing the Prism Coupling of Laser Light from the Back Side of the Gold Film and the Collection of Surface Plasmon Enhanced Fluorescence Normal to the Gold Surface
Figure 1. Surface plasmon resonance curve and surface plasmon enhanced fluorescence signal for a self-assembled monolayer of 11amino-1-undecanethiol on a gold film (dotted lines). The solid lines show the SPR and SPFS signals for the same film after the deposition of a layer of CdZnSe quantum dots.
EDC-ester groups at the surface. After 1 min, the solution was allowed to flow through the Teflon flow cell and across the 11amino-1-undecanethiol SAM at a rate of 1 mL/min. The EDC/ q-dot solution was recycled around the closed flow system until no more binding was observed. Ethanolamine (0.1 M) was introduced to the cell to remove any unreacted ester. Finally, the cell was rinsed through with more HEPES buffer. Results and Discussion A laser can be used to excite surface plasmon modes at the interface between a metal and a dielectric medium. The resulting optical field decays exponentially into both the metal and the dielectric, with the momentum of the surface plasmons being sensitive to changes in refractive index of the dielectric medium.16 This is the basis of surface plasmon resonance spectroscopy (SPR) where changes in coupling angle (θ, see Scheme 1) can be related to changes in refractive index (i.e., the presence of thin adlayers) at the surface of the metal. If a chromophore is placed within the optical field close to the metal surface it can be excited by the surface plasmon field. In surface plasmon enhanced fluorescence spectroscopy (SPFS) the emitted photons are collected and counted, and most commonly the signal is used to detect the presence of fluorescently labeled molecules binding at the metal surface. A comprehensive review of SPR is given in ref 16 and the technique of SPFS is discussed in detail in ref 17. As discussed in the Introduction, quantum dots show fluorescence enhancement when placed near rough metal surfaces when compared to quantum dots on atomically flat metal or nonmetallic surfaces. In this case the evaporated gold film had an intrinsic surface roughness of ∼20-30 nm and a strong surface plasmon enhanced fluorescence signal was measured despite the fact that the quantum dots were only ∼1.3 nm away from the gold. The quantum dots were CdxZn1-xSe alloys with ∼5.5 nm diameter and band edge emission centered on 614 nm. Surface plasmons were excited with an orange (λ ) 594.6 nm) laser and the resulting surface enhanced fluorescence from the QDs was passed through a 611 nm band-pass filter to
exclude scattered laser light before being incident on a photomultiplier tube. The formation of submonolayer arrays of quantum dots by the covalent attachment of carboxylic capped dots to amineterminated self-assembled monolayers has been described in detail elsewhere.15 Briefly, the ligand shell of the CdZnSe QDs contains 11-mercaptoundecanoic acid, where the thiol binds to the QD surface and the acid groups face outward. The gold film is modified with 11-amino-1-undecanethiol to form a selfassembled monolayer with amine groups at the surface. 1-Ethyl3-(3-dimethylaminopropyl)carbodiimide is reacted with the carboxylic groups surrounding the QD to form an activated ester. When the activated QDs are introduced to the amine groups a covalent amide bond forms with urea as the leaving group. A submonolayer of covalently bound QDs is formed. Figure 1 shows the SPR and SPFS response before and after binding CdZnSe nanoparticles to the SAM on the gold electrode. The SPR curve can be seen to shift to the right as the QDs bind to the gold surface. A strong surface enhanced fluorescence signal was measured, with more than a 10-fold increase in fluorescence relative to the background. Dark Electrochemistry. Figure 2 shows typical cyclic voltammograms carried out before and after the surface attachment of 614 nm emitting CdZnSe quantum dots. The scan rate was 10 mV s-1. When the quantum dots are present an increased reduction current relative to the background is seen at more negative potentials. All electrochemistry was carried out in water and in the presence of oxygen to be relevant to the normal biosensing environment. Electrochemical Response under Chopped Illumination. The electrochemical response of a CdZnSe film was investigated under chopped illumination from a high intensity lumiled LED at 470 nm. Figure 3 shows a linear sweep voltammogram between 0 and -0.7 V at a scan rate of 1 mV s-1. The light was chopped on and off manually every 15 s. Positive of -0.44 V (versus Ag/AgCl) an anodic photocurrent was measured under illumination. The excess current is due to optically excited electrons in the quantum dot being transferred to the gold and the hole in the quantum dot being filled by electron donation
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Figure 2. Cyclic voltammograms before (dashed line) and after (solid line) attachment of quantum dots; the scan rate was 10 mV s-1.
Figure 4. SPFS signals from the QD film as different potentials were applied to the gold working electrode relative to a silver/silver chloride reference electrode. The decrease in the SPFS signal with increasingly negative potentials can be seen. The inset shows the background fluorescence for an identical film without a CdZnSe QD layer; in this case no difference in the SPFS signal was seen with applied potential.
Figure 3. Linear sweep voltammogram of a CdZnSe nanoparticle film on a gold electrode under chopped illumination at 470 nm. Positive of -0.4V an enhanced positive current was seen upon illumination; negative of -0.4 V an enhanced negative photocurrent is seen upon illumination. The scan rate was 1 mV/s.
from sulfite ions in the solution. At potentials negative of -0.44 V a negative photocurrent is seen under illumination, this must correspond to more electrons flowing from the electrode to the surface attached film when the light is switched on. The enhanced current could be due to a conductivity effect, where the conductivity of the film increases under illumination. Alternatively this process could be due to a photoinduced reduction of the ligands or other film components.18 It has previously been reported that under illumination absorption of ambient oxygen to the surface of CdSe quantum dots can lead to efficient trapping of the photogenerated electrons and hence an excess of holes in the QD which could be filled by donation of electrons from the electrode if the applied potential was sufficiently negative. Such a mechanism could also explain the negative photocurrents when the light is switched on.19-21 Potential Induced Changes in the Surface Enhanced Fluorescence Signal. The effect of the electrode potential on the surface plasmon enhanced fluorescence signal was investigated both as a function of incident angle and by kinetic monitoring of the fluorescence signal at a single incident angle. Figure 4 shows the surface enhanced fluorescence response at applied potentials between 0 and -0.7 V; care was taken to measure the scan after a steady state reduction current was achieved (>500 s). It was not possible to measure negative of
Figure 5. Fluorescence maxima (from Figure 4) as a function of applied potetential.
-0.7 as at -0.75 V the self-assembled monolayer was reductively desorbed and the surface film destroyed. The application of increasingly negative potentials caused a concurrent decrease in the SPFS signal. The decrease was reversible; returning the cell to 0 V (versus Ag/AgCl) or to open circuit resulted in the recovery of the original fluorescence intensity. Figure 5 shows the decrease in the maximum fluorescence as a function of potential. A fairly linear reduction in fluorescence with increasing potential was obtained. This experiment was repeated three times with different surface loadings of quantum dots; in all cases a reversible decrease in the fluorescence intensity was measured as negative potentials were applied.22 The background fluorescence showed no potential dependence and remained constant all the way to -0.7 V (inset Figure 4). A reversible change in the fluorescence intensity was only observed when the QDs were present. This strongly suggests that the changes are not due to changes in surface plasmon coupling with the gold layer as the potential is changed.
Surface Immobilized CdZnSe Quantum Dots
Figure 6. Potential step experiment where the applied potential was stepped from 0 to -0.7 V (at t ) 0) and then back to 0 V (at t ) 1000): above the current response with applied potential (versus Ag/ AgCl); below the decay and recovery of the fluorescence signal as the applied potential was changed.
It is worth noting that the reduction in fluorescence with applied potential is seen at relatively low potentials, certainly at energies much lower than the lowest occupied molecular orbital (LUMO) of the quantum dot. The LUMO energy can be estimated from Figure 3 as at potentials positive of -0.44 V photoexcited electrons can be injected from the LUMO to the electrode and a positive photocurrent is measured. The positive photocurrent disappears negative of -0.44 V suggesting that the electrode Fermi-level is then at a similar energy to the LUMO and the driving force for LUMO-electrode electron transfer disappears. It is possible that the decrease in fluorescence positive of -0.44 V is due to the reduction of surface sites on the quantum dots where the injected electron at the surface suppresses charge separation and radiative emission from the QD; a similar mechanism has been proposed for PL quenching in CdSe QDs in organic solvents.4-6 Figure 6 shows the kinetic response at a single angle of incidence (the angle at which the SPFS signal was at a maximum) as the potential was stepped from 0 to -0.7 V and back to 0 V. The potential induced decrease in fluorescence was relatively slow, with steady state being reached after 500 s. Figure 7 shows the bleach recovery when the cell was either disconnected or returned to 0 V bias. In both cases the fluorescence intensity recovered at a similar rate over a similar time scale (∼750 s), although initially the bleach recovery is slightly faster in the case where the cell is switched to 0 V rather than to open circuit. Bleach recovery both by reoxidation of the QD film and a return to open circuit supports the theory that the fluorescence is quenched by reduced sites at the surface of the quantum dots. These sites can be either oxidized electrochemically or oxidized by electron acceptors in the solution at open circuit to restore the fluorescence signal from the QD. Jha et al.6 measured photoluminescence bleaching of CdSe nanoparticle films drop cast onto ITO electrodes. The measurements were carried out in anhydrous DMF with 0.1 M tetrabutylammonium perchlorate. In the experiments with CdSe films it was postulated that injecting electrons into the nanoparticles created “quenching centers”, e.g., surface redox sites that dominated fluorescence quenching and had to be oxidized
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Figure 7. Fluorescence recovery when the applied potential is switched from -0.7 to 0 V (solid line) or from -0.7 V to open circuit (dashed line).
by solution species before the photoluminescence could recover. The result was attributed to the removal of selenium from the surface, which when reduced was thought to be the main cause of quenching sites. A review by Itmar et al. in 2007 details a number of biosensing approaches and methodologies using QDs including CdSe QDs for fluorometric sensing of analytes including maltose and nitrate.23 The possibility of using an applied electrode potential to control the fluorescence emission of a tethered QD raises the possibility of creating sensor arrays with greater substrate specificity and/or sensitivity. Up to now the majority of systems which have shown potential control of fluorescence from QDs have required working in organic solvents, which is nonideal for most bio and biological sensing applications. In the research described in this paper, we show that such electrode potential control of QD fluorescence can also be achieved in an aqueous environment by close control of coupling conditions and QD chemistry leading to the possibility of using such structures for enhanced detection of biological and biorelated molecules. Conclusions CdZnSe alloyed nanoparticles have been covalently attached to gold electrodes. A clear surface plasmon enhanced fluorescence signal was measured and the intensity of the fluorescence could be tuned reversibly by applying an electrochemical signal to the gold electrode. It is postulated that the changes in fluorescence are due to the reduction of surface redox sites on the quantum dots. Unlike most fundamental studies of the effect of an electrochemical potential on the optical properties of quantum dots, this study was carried out in an aqueous environment and in the presence of oxygen. The quantum dot fluorescence emission was highly dependent on the applied electrochemical potential; the results are therefore relevant to groups using quantum dots in the vicinity of conducting surfaces for applications in biosensing. Acknowledgment. P.J.C. thanks the Alexander Von Humboldt Stiftung and Research Councils UK (RCUK) for funding.
6008 J. Phys. Chem. C, Vol. 113, No. 15, 2009 References and Notes (1) Robelek, R.; Stefani, F. D.; Knoll, W. Phys. Status Solidi 2006, 203, 3468–3475. (2) Portney, N. G.; Ozkan, M. Anal. Bioanal. Chem. 2006, 384, 620– 630. (3) Houpten, A. J.; Vanmaelkelberg, D J. Phys. Chem. B 2005, 109, 19634–19642. (4) Wang, C.; Wehrenberg, B. L.; Woo, C. Y.; Guyot-Sionnest, P. J. Phys. Chem. B 2004, 108, 9027–9031. (5) Shim, M.; Wang, C.; Guyot-Sionnest, P. J. Phys. Chem. B 2001, 105, 2369–2373. (6) Jha, P. P.; Guyot-Sionnest, P. J. Phys. Chem. C 2007, 111, 15440– 15445. (7) Cordeo, S. R.; Carson, P. J.; Estabrook, R. A.; Strouse, G. F.; Buratto, S. K. J. Phys. Chem. B 2000, 104, 12137–12142. (8) Myung, N.; Bae, Y.; Bard, A. J. Nano Lett. 2003, 3, 747–749. (9) Ito, Y.; Matsuda, K.; Kanemitsu, Y. Phys. ReV. B 2007, 75, 033309/ 1–033309/4. (10) Okamoto, K.; Vyawahare, S.; Scherer, A. J. Opt. Soc. Am. 2006, 1674–1678. (11) Komarala, V. K.; Rakovich, Y. P.; Bradley, A. L.; Byrne, S. J.; Gun’ko, Y. K.; Gaponik, N.; Eychmueller, A. Appl. Phys. Lett. 2006, 89, 253118/1–25118/2.
Cameron et al. (12) Song, J.-H.; Atay, T.; Shi, S.; Urabe, H.; Nurmikko, A. V. Nano Lett. 2005, 5, 1557–1561. (13) Malicka, J.; Jiang, W.; Fischer, H.; Chan, W. C. W.; Gryczynski, Z.; Grudzinski, W.; Lakowicz, J. R. J. Phys. Chem. B 2005, 109, 1088–1093. (14) Zhong, X.; Han, M.; Dong, Z.; White, T. J.; Knoll, W. J. Am. Chem. Soc. 2003, 125, 8589–8594. (15) Cameron, P. J.; Zhong, X.; Knoll, W. J. Phys. Chem. C 2007, 111, 10313–10319. (16) Knoll, W. Annu. ReV. Phys. Chem. 1998, 49, 569–638. (17) Neumann, T.; Johansson, M.; Kambhampati, D.; Knoll, W. AdV. Funct. Mater. 2002, 12, 575–586. (18) Aldana, J.; Wang, Y. A.; Peng, X. J. Am. Chem. Soc. 2001, 123 (36), 8844–8850. (19) Koberling, F.; Mews, A.; Basche´, T. AdV. Mater. 2001, 13, 672– 676. (20) Walker, G. W.; Sundar, V. C.; Rudinski, C. M.; Wun, A. W.; Bawendi, M. G.; Nocera, D. G. Appl. Phys. Lett. 2003, 83, 3555–3557. (21) Pradhan, S.; Chen, S.; Wang, S.; Zou, J.; Kauzlarich, S. M.; Louie, A. Y. Langmuir 2006, 22, 787–793. (22) Knoll, W.; Cameron, P.; Caminade, A. M.; Feng, C. L.; Kim, D. H.; Kreiter, M.; Majoral, J. P.; Mu¨llen, K.; Rocholz, H.; Shumaker-Parry, J.; Steinhart, M.; Zhong, X. Proc. of SPIE 2007, 6768, 67680T, 1–10. (23) Willner, I.; Basnar, B.; Willner, B. FEBS J. 2007, 274, 302–309.
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