Quantifying Quantum Dots through Förster Resonant Energy Transfer

ARTICLE pubs.acs.org/JPCC

€rster Resonant Energy Transfer Quantifying Quantum Dots through Fo Preston T. Snee,* Christina M. Tyrakowski, Leah E. Page, Adela Isovic, and Ali M. Jawaid Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607-7061, United States

bS Supporting Information ABSTRACT: Semiconductor nanocrystals (NCs or quantum dots) have significant potential for use in a variety of applications from renewable energy generation to biological imaging. Modern methods of colloidal synthesis can be used to create crystalline materials with tight size distributions; this assures high quantum yields and narrow emission profiles in the case of direct-bandgap semiconductors. The optical properties of NCs may also be tuned with size due to quantum confinement effects. Quantum confinement also creates problems when characterizing nanomaterials; specifically, the absorptivity of a sample is a function of the size and structure of the quantum dots. We demonstrate here a simple method for determining the molar absorptivity of aqueous CdSe/CdZnS NCs through F€orster resonant energy transfer. Energy transfer from NC donors to dye acceptors was measured and modeled using standard F€ orster theory incorporating Poissonian statistics to calculate the acceptor/donor ratio leading to a direct determination of the NC concentration. This process is significantly more simple than standard methods for calculating the size-dependent Beer’s law coefficient of nanomaterials and can be applied to heterogeneous quantum confined systems. The results also yield surprising insight into the internal structure of water-soluble polymer-coated core/shell quantum dots.

1. INTRODUCTION Developing applications for semiconductor nanocrystals (NCs) has become a significant endeavor in the physical and biological sciences. Recent results have demonstrated advances in renewable energy generation using heterogeneous NCs;1 furthermore, the high quantum yields of core/shell directbandgap materials have created inroads for the use of nanocrystals in display technology and imaging.2 Early work in the biological arena established the utility of NCs for imaging,3,4 and now basic biological phenomena are being elucidated using the quantum dot toolkit.5 The physical properties of NCs have also been extensively explored; recent research has demonstrated interesting magnetic phenomenon in doped nanomaterials,6 while breakthroughs are still being made in understanding the fluorescence intermittency of quantum dots.7 The possibility that nanotechnology may become ubiquitous has also stimulated research on the effects that quantum dots may have on the environment.8,9 A significant property of quantum confined systems is the ability to tune absorption and/or emission spectra through manipulation of the size, shape, composition, and internal structure of nanomaterials.10 14 While a useful and interesting characteristic, the size-dependent optical properties also engender a very significant issue concerning the characterization of nanomaterials, namely that the molar absorptivity is also a function of size.15,16 The determination of the concentration of a solution of NCs is a nontrivial task. Generally, one must produce a homogeneous sample, characterize the volume of the NCs through extensive electron microscopy analysis, and determine the ion content with elemental analysis.16 The r 2011 American Chemical Society

quantity of NCs in a particular sample is the total ion content divided by the average number of the same element per nanocrystal. Alternatively, one may synthesize a batch of NCs and calculate the yield gravimetrically (or assume 100% efficiency); dividing the moles of consumed semiconductor precursors by the number of the same element per NC (again based on volume) can also quantify the number of quantum dots.15 In either case, a regression to calculate the size-dependent molar absorptivity for a given material system can be determined by repeating such processes over a well-defined distribution of sizes. Obviously, measuring the molar absorptivity of just a single material system is highly time-consuming. To complicate matters further, the absorptivity of heterogeneous core/shell systems, such as the ubiquitous CdSe/ZnS,17 changes as a function of shell thickness.18 Even worse is the fact that the shell may be alloyed; as is well-known and demonstrated here, doping the shell with cadmium creates brighter CdSe/CdZnS NCs19 yet alters the molar absorptivity. Thus, creating a multidimensional regression for molar NC absorptivity as a function of core and shell compositions, as well as core diameter and shell thickness, is a seemingly impossible task. We demonstrate here a simple method to determine the concentration of an aqueous dispersion of nanocrystals using F€orster resonant energy transfer (FRET)20 from a NC donor to Received: June 21, 2011 Revised: August 22, 2011 Published: August 31, 2011 19578

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an organic dye acceptor.21,22 First, the characteristic FRET length scale (R0) from the NC to the dye is determined. Next, the emission of the NC is measured as a function of the addition of the dye; the FRET efficiency is then calculated by the quenching of the NC donor. As the FRET efficiency is a function of the acceptor to donor ratio (after accounting for Poissonian statistics), the data are fit to F€orster Theory to calculate the concentration of NCs in solution. The optical cross section can then be determined at any convenient wavelength. Note that we do not need to make any estimation of the size of the NCs, their elemental composition, or the reaction yields, to successfully perform these measurements.

2. EXPERIMENTAL SECTION All chemicals were obtained from commercial sources and were used as received unless noted. 2.1. Instrumentation. 1H NMR spectra were recorded on a Bruker Avance DRX 400 NMR Spectrometer. Optical spectra were measured with a Varian Cary 300 Bio UV/vis spectrophotometer (absorbance) and a custom-designed Fluorolog from HORIBA Jobin-Yvon (photoluminescence). The absorbance of all solutions was kept near or below 0.1 OD at the excited wavelength to avoid inner-filtering effects; however, the concentration of dye cannot be so low such that its absorbance is difficult to measure. Likewise, the NC emission must be strong enough to produce quantifiable data to determine the FRET efficiency. A JEOL JEM-3010 operating at 300 kV was used for transmission electron microscopy (TEM) measurements. 2.2. CdSe/CdZnS Synthesis. Core CdSe and core shell CdSe/ ZnS NCs were synthesized according to previously published protocols;23,24 the procedure will be restated here for completeness. A 3-neck 25 mL glass round-bottom flask was loaded with 6.0 g of distilled trioctylphosphine oxide (TOPO, originally 90%, Sigma-Aldrich, see ref 25 for details), 0.5 g of tetradecylphosphonic acid (TDPA, prepared according to ref 26), 6.0 mL of trioctylphosphine (TOP, 97%, Strem), and 0.2623 g of cadmium acetate dihydrate (99.6%, Fisher Scientific). The solution was degassed at ∼80 100 °C, heated to 250 °C under an inert N2 atmosphere, cooled back to ∼80 100 °C, and degassed again. Next, the solution temperature was raised to 350 °C under N2 and 1.5 mL of 1 M trioctylphosphine selenide (prepared from TOP and selenium shot) was quickly injected into the solution while rapidly stirring. The heating mantle was lowered and the solution was allowed to cool to room temperature. Samples were processed by addition of a small amount of isopropanol followed by methanol to induce precipitation. The supernatant is discarded. The core NCs are then dispersed in a known quantity of hexane (∼4 mL). A 25 μL quantity was diluted to 2.5 mL in hexane and the absorption was measured. The total moles of the core NCs was then back-calculated from these data using Leatherdale’s folmula.15 The sample was split into two and each was separately overcoated. Each quantity was separately dispersed into a degassed solution of 10 g distilled TOPO, 0.5 g tetradecylphosphonic acid, and varying amounts of cadmium acetate dihydrate; sample “A” had 8 mg of the cadmium precursor while “B” had 25 mg in the degassed solution. Hexane was removed under vacuum over the course of 1 h. Next, the solution was heated to 160 °C and two precursor solutions were slowly added over the course of ∼2 h using a syringe injector. One solution had 100 mg of diethyl zinc (Strem) in 3 mL TOP while the other had 110 mg bis-trimethylsilyl sulfide

Figure 1. Absorption spectrum of core CdSe and core/shell CdSe/ CdZnS NCs with the stoichiometry of the shell growth precursors indicated (spectra are normalized to their first absorption feature). The absorption red-shift is strongly dependent on the amount of cadmium in the shell. Inset: a phenomenological model of the palmitic acidrhodamine B dye interacting with a polymer-coated core/shell NC. The stoichiometry of the shell reflects that of the shell precursor solutions; the actual composition of the shell may vary which is unfortunately difficult to assess, see ref 19 for details.

(purum, Sigma-Aldrich) in 3 mL TOP. After the solutions have been added, the CdSe/CdZnS were stored in growth solution. We calculate the number of NCs per gram of growth solution by assuming no NCs were lost in the processing. While we were not able to quantify the cadmium content of the shells after growth, there are significant differences between samples A and B which we attribute to the quantity of cadmium precursors used in the overcoating process as discussed below. These NCs were water solubilized with a 40% octylamine modified poly(acrylic acid) polymer,27 see ref 28 for details on this procedure. 2.3. Palmitic Acid-Functional Rhodamine B Piperazine Synthesis. Rhodamine B piperazine was synthesized using the protocol described in ref 29. Approximately 20 mg of palmitic acid (Sigma Life Science, g 99%) was added to 20 mg of rhodamine B piperazine and 35 mg of benzotriazole-1-yl-oxytris-(dimethylamino)-phosphonium hexafluorophosphate (BOP, Novabiochem) in 10 mL of dry DMF (Acros, extra dry); after addition of a few drops of triethylamine (Fluka, puriss), the solution was stirred overnight. The next day, the solvent was removed in vacuum and the residue was added to dry ethyl acetate. Hexane is added until a white precipitate is observed, which is discarded after centrifugation. Next, the product is dried and dissolved in distilled methanol and purified over activated alumina. After drying, the product was characterized with 1H NMR spectroscopy as 85% pure with BOP accounting for the remaining material present; 16 mg of dye was weighed providing a reaction yield of 46%. This mass of dye was dissolved and diluted into DMF; small quantities of dye were added to an aqueous NC dispersion in increasing amounts to calculate a molar absorptivity of ε565 nm = 39 ( 3  103 M 1 cm 1 using a linear regression. The quantum yield (QY) of the dye in neat water is only 10%; the low value is likely due to dye aggregation and self-quenching. Upon addition of 40% octylamine-modified poly(acrylic acid), the same polymer used to water-solubilize NCs, the QY rises to 39.5%. 19579

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Table 1. Summary of Results sample ID

absa

QYb

R0 (nm)

R (nm)c

A

511

0.17

4.2

3.1

3.6  10

7

4.5  10

7

4.1

2.9  10

7

2.7  10

7

B

527

0.29

4.9

[NC]d (M)

εmax (M

[NC]FRETe (M)

1

cm 1)f

ε511 = 1.9  105 ε527 = 2.9  105

The wavelength of the first absorption feature; the core value is 494.5 nm. The quantum yield of the NCs in water. From the fit to the FRET efficiency data. d The concentration of NCs according to the number of core CdSe quantum dots produced, and accounting for that mass through processing. e The concentration of NCs measuring using FRET. f The molar absorptivity at the first absorption feature determined from the FRET method. a

Figure 2. A. TEM micrograph of sample A shows quasi-spherical particles with an average aspect ratio of 1.4 ( 0.2. B. Larger “bulletshaped” NCs are produced when the cadmium content is raised to 12% within the shell growth solution as in sample B. The aspect ratio increases to 1.9 ( 0.2.

3. RESULTS AND DISCUSSION 3.1. Core/Shell NC Structure and Optical Spectra. The use of cadmium in the shell-growth process affects the optical spectra of overcoated NCs as shown in Figure 1. Adding 4% (sample “A”) to 12% (sample “B”) cadmium relative to zinc red-shifts the first absorption feature by 0.08 and 0.15 eV, respectively, which in turn wipes out knowledge of the size of the original core NC based on the location of the first absorption feature.15,16 The quantum yield of the NCs in water nearly doubles with the addition of extra cadmium in the shell growth medium as shown in Table 1; nearly unit quantum yields may also be obtained when the cadmium content of the shell is raised to ∼50% as we demonstrated recently,30 see the Supporting Information for examples and methods. TEM micrographs of the samples reveal Cd-dependent morphological changes during overcoating as shown in Figure 2. Coating the initial 2.3 nm core16 with a 4% Cd to Zn ratio produces prolate NCs with a long diameter of 4.0 ( 0.5 and 2.8 ( 0.4 nm on the short axis. When the cadmium content was raised to 12% relative to zinc, the NCs do not become thicker (2.5 ( 0.2 nm); however, they elongate to 4.7 ( 0.4 nm. As shown in the high resolution TEM of Figure 2B, the NCs also develop a “bullet-shape” appearance as has been observed in core/cadmium-rich shell NCs previously19 and is likely the result of the anisotropic Wurtzite crystal structure of the CdSe core as discussed in ref 19. This anisotropy also appears to affect the structure of the polymer-encapsulated materials as discussed below. 3.2. NC-Dye Coupled Chromophores. We have been reporting NC concentrations based on the initial calculated core molarity and assuming no losses from synthesizing core/shell NCs and water-solubilizing them;31 clearly this is not a robust assumption and not applicable to other, uncharacterized core

b

c

materials. The present method of examining energy transfer from a NC donor to a dye acceptor was developed to address this issue; unfortunately, the functionalization of aqueous NCs with dyes is a nontrivial task due to the colloidal nature of the nanocrystal dispersion. Our group has recently reported several methods to conjugate a variety of chemical and biological species to watersoluble nanocrystals through the use of electrostatically neutral carboxylic acid activators or via thiol-maleimide coupling.28,31,32 However, less than unitary reaction efficiencies predicate the need for dialysis to remove unreacted precursors; we have observed that such processing can lower the quantum yield (QY) of the sample. For this study, we developed a method of conjugation with ∼97.5% efficiency that does not require the postprocessing that would result in NC quenching misinterpreted as being due to energy transfer. We noted previously that slightly hydrophobic dyes, such as BODIPYs, can nonspecifically adsorb to the surface of polymer-encapsulated NCs such that they cannot be removed with dialysis.33 This led us to synthesize a palmitic acid derivative of rhodamine B piperazine shown in Figure 1.29 This dye can be dissolved in DMF at a high concentration and subsequently mixed with an aqueous NC dispersion; the interaction with the NC-centered micelle is strong enough that the dye will not separate significantly from the NCs with dialysis (see the Supporting Information). The dye is also a good FRET acceptor to green emitting nanocrystals as shown in Figure 3A. This allows for the NCs to be quenched through energy transfer and not by the extra processing that other conjugation methods require. The data for this work was obtained by adding single microliter quantities of hydrophobic rhodamine B piperazine dye in DMF to stock solutions of samples A and B in water, allowing the mixture to equilibrate for several minutes, and then measuring the absorption and emission spectra as shown in Figure 3B. Care was taken to keep the absorption of the samples below 0.1 OD; furthermore, the samples were excited at a minimum in the dye absorption at 440 nm. Some solutions were filtered if any indication of scattering appeared in the absorption; see the Supporting Information for data from other samples. 3.3. FRET Efficiency Calculations. The purpose of this work is to demonstrate that FRET efficiency data can be used to calculate the average acceptor/donor ratio; knowledge of the absolute concentration of one leads to the immediate determination of the other. In this regard, our study is similar to a recent report by Morris-Cohen et al.34 that used ultrafast laser spectroscopy to quantify the average number of electron-withdrawing ligands (a viologen derivative) bound to the surface of semiconductor CdS NCs. These data allowed the group to calculate the association constant (K a) of the ligand. Our method represents a more chemically oriented technique that produces similar results, and while we avoid the necessity of making sophisticated ultrafast laser measurements, the synthesis of the 19580

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Figure 3. A. Visible changes in the emission of sample A (NC) with addition of palmitic acid functionalized rhodamine B piperazine (NC-dye) are evident to the eye, see Figures S4 and S5 of the Supporting Information. B. The emission spectra of sample B as a function of the addition of the same hydrophobic dye. Inset: Absorption spectra of the same, marked with the excitation wavelength (440 nm).

Figure 4. FRET efficiency as calculated by the quenching of the NC donor (red) and fit to the data (blue). The dye/NC ratios were determined by the fit to the FRET efficiency data. Inset: The best fit without incorporation of Poissonian statistics.

hydrophobically modified rhodamine B piperazine is also a nontrivial task. The FRET efficiency was determined by correcting the emission for self-absorption and then fitting the spectra to NC and dye components using Matlab; the quenching of the NC compared to the initial emission was used to calculate the FRET efficiency. An example for sample B is shown in Figure 4. These data are n λ 6 6 6 fit to E(λ,R) = ∑∞ n=0(λ e /n!)[nR0 /(nR0 + R )], where E is the FRET efficiency (known to have a normal 1/R6 dependence in the case of NC-dye couples),35 n is a whole-number acceptorto-donor ratio while λ is the average of the same, R is the NC-to-dye distance, and the characteristic FRET length scale R0 was calculated from the normalized emission profile of the NCs and the molar absorption spectrum of the hydrophobic dye (see the SI).36 Several recent reports have demonstrated the necessity of using Poissonian statistics to account for the distribution of whole-number dye/NC ratios.34,37,38 Such

effects were incorporated into the fit by summing the FRET efficiencies for probability-weighted whole-number acceptor donor ratios (n) where the average acceptor donor ratio (λ) was a parameter of the fit along with R, the NC-to-dye distance. As shown in Figure 4, these data are fit to this procedure very well; furthermore, the inclusion of Poissonian statistics is necessary as indicated by the lack of reproduction of the FRET efficiency if the randomness of acceptor donor ratios is not considered (inset). Although we are simultaneously fitting two parameters, the use of multiple data points (∼11 12 dye/NC ratios) allows for the unique determination of acceptor/donor ratio and R as demonstrated in the Supporting Information. The acceptor/donor ratio then allows us to calculate the original donor concentration given that the dye was added in known molar increments to a 2.5 mL solution of aqueous core/shell NCs. These data are summarized in Table 1. Overall, the concentrations predicted from the calculation of the original number of core nanocrystals are very similar to those determined from FRET data. Unfortunately, there is no sound way to evaluate which method is more accurate; however, the results demonstrate that a FRET-based analysis is capable of predicting reasonable values for NC concentrations, saving time and energy. We further note that without characterization of the original homogeneous core NCs, the quantification of core/shell CdSe/ CdZnS NCs with traditional elemental analysis is impossible due to the inability to differentiate the epitaxial core/shell boundary,19 significant red-shift of the first absorption feature, and similarity of the core/shell elemental compositions. 3.4. “Bubble” Model of Polymer-Coated NCs. It is of interest to note that the calculated NC-chromophore distance (R) increases from sample A to B by 1 nm. This is similar to the change in the length of the prolate NCs; as discussed above, TEM analysis revealed that sample B is 0.7 nm longer than sample A, but they have similar widths on two other axes. Thus, it is perplexing that a single asymmetric axis has a significant effect on the calculated NC-dye distance. To account for this observation, we propose that the encapsulating 40% octylamine-modified poly(acrylic acid) amphiphilic polymer functions like a “bubble” around the hydrophobic NC and maintains an overall spherical symmetry. The conformational flexibility to do so may be the result of the relatively low molecular weight of the amphiphilic polymers (∼2910 Da) used to water-solubilize CdSe/CdZnS NCs and the fact that several hundred polymers 19581

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The Journal of Physical Chemistry C coat a single NC.31 Thus, these amphiphiles may have large degrees of freedom to minimize hydrophobic hydrophilic interactions and form spherical micelles, the diameter of which is determined by the single long axis of the asymmetric NC. This in turn dictates the NC-to-dye coupled chromophore distance.

4. CONCLUSION In summary, we have shown that the absorption spectra of core/shell NCs are altered from the core and that the molar absorptivity changes as a result. Our previously reported method of methodically tracking the number of NCs from the original growth solution to the aqueous product is a good approximation, yet presumes that no loss of material occurs in any step after the initial synthesis. The FRET-based method presented here offers a simple solution; the use of a hydrophobically modified dye that strongly adsorbs to the aqueous polymer-coated NCs, without the need for processing, and causes energy-transfer quenching that can be modeled to determine the absolute NC concentration. The method also reveals structural information about the polymer-coated NCs. While we have studied only core/shell CdSe/CdZnS NCs, we believe that this method may be generalized to other novel systems that have never been characterized using traditional methods. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional spectral and characterization data, as well as additional experimental protocols and analysis procedures. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the University of Illinois at Chicago with partial support from the UIC Chancellor’s Discovery Fund and a PRF award (50859-ND10) from the ACS. ’ REFERENCES

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