Chemical Defects in the Highly Fluorescent Conjugated Polymer Dots

Nov 11, 2010 - Patrick C. Tapping , Scott N. Clafton , Kyra N. Schwarz , Tak W. Kee , and .... T Fitzgerald , William M Gallagher , Patrick Kiely , Ga...
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Chemical Defects in the Highly Fluorescent Conjugated Polymer Dots Scott N. Clafton,† David A. Beattie,‡ Agnieszka Mierczynska-Vasilev,‡ Robert G. Acres,‡ Alan C. Morgan,† and Tak W. Kee*,† †

‡

Department of Chemistry, University of Adelaide, Adelaide, South Australia 5005, Australia, and Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, Mawson Lakes, Adelaide SA 5095, Australia Received August 2, 2010. Revised Manuscript Received October 28, 2010

We present strong evidence for the oxidation of conjugated polymers in the formation of conjugated polymer dots (CPdots) using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Although recent studies show that folding of the polymer chain into a compact 3D structure is involved in the formation of these nanoparticles, the process by which these intrinsically hydrophobic nanoscale particles circumvent aggregation in water is still not well understood. Zeta potential results show that these dots have a negatively charged surface at neutral pH, with a zeta potential and surface charge density of approximately -40 mV and (1.39 - 1.70)10-2 C/m2, respectively. In addition, quantitative elemental analysis of CPdots indicates that oxygen composes 7-13% of these nanoparticles. The overall results support the presence of chemical defects in forming a hydrophilic surface of CPdots. As a consequence, the charged surface contributes to inhibiting the aggregation of CPdots in water, leading to colloidal stability.

Introduction We report the presence of chemical defects in conjugated polymer nanoparticles, or “conjugated polymer dots” (CPdots), and the role of oxidation in the colloidal stability of these highly fluorescent nanomaterials. We propose that the oxidation of conjugated polymers results in partially ionized defects, which in turn stabilize the nanoparticle suspension. The CPdots are emerging as a novel class of fluorescent nanoparticles.1-8 The highly luminescent CPdots have an excellent potential to be functional materials for light-emitting displays and robust probes for fluorescence imaging. These nanoparticles exhibit desirable characteristics for cellular imaging, including a high fluorescence quantum yield, good photostability, and a small size. Wu et al. have developed a simple method to prepare 10-15 nm CPdots using several conjugated polymers and have demonstrated a number of applications.4-8 Most recently, a study showed that the remarkable brightness of the CPdots, with a fluorescence cross section of 2.210-13 cm2, is the enabling factor in video rate particle tracking at ∼1 nm resolution.8 The CPdots have long-term colloidal stability, showing no sign of aggregation or precipitation for weeks or longer. The formation of CPdots has been described as the collapse of hydrophobic polymer chains in water.5 Simulations show that the collapse of a single chain results in a roughly spherical, globular conformation as a consequence of a strong intrachain hydrophobic attraction and a large amount *Corresponding author. E-mail: [email protected]. (1) Palacios, R. E.; Fan, F.-R. F.; Grey, J. K.; Suk, J.; Bard, A. J.; Barbara, P. F. Nat. Mater. 2007, 6, 680. (2) Chang, Y.-L.; Palacios, R. E.; Fan, F.-R. F.; Bard, A. J.; Barbara, P. F. J. Am. Chem. Soc. 2008, 130, 8906. (3) Grey, J. K.; Kim, D. Y.; Norris, B. C.; Miller, W. L.; Barbara, P. F. J. Phys. Chem. B 2006, 110, 25568. (4) Wu, C.; Szymanski, C.; Cain, Z.; McNeill, J. J. Am. Chem. Soc. 2007, 129, 12904. (5) Wu, C.; Szymanski, C.; McNeill, J. Langmuir 2006, 22, 2956. (6) Wu, C.; Bull, B.; Szymanski, C.; Christensen, K.; McNeill, J. ACS Nano 2008, 2, 2415. (7) Wu, C. F.; Bull, B.; Christensen, K.; McNeill, J. Angew. Chem., Int. Ed. 2009, 48, 2741. (8) Yu, J.; Wu, C.; Sahu, S. P.; Fernando, L. P.; Szymanski, C.; McNeill, J. J. Am. Chem. Soc. 2009, 131, 18410.

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of interfacial surface tension between the polymer and water.9,10 Recent studies using atomic force microscopy (AFM) and scanning electron microscopy confirm the spherical structure of CPdots.5,11 Although simulations have provided important clues in the formation of CPdots, the means by which these hydrophobic nanoparticles achieve long-term colloidal stability as an aqueous suspension is still not fully understood. In this study, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and elemental analysis were used to detect and quantify the presence of chemical defects in CPdots. In addition, surface charge measurements confirm that CPdots have a negatively charged exterior at neutral pH. The overall result indicates that the negatively charged chemical defects are present on the surface, inhibiting the aggregation of CPdots.

Experimental Section Materials. The conjugated polymers used in this study are poly(9,9-dihexylfluorenyl-2,7-diyl) (PDHF, MW = 103 000), poly[{9,9-dioctyl-2,7-divinylenefluorenylene}-alt-co-{2-methoxy-5(2-ethylhexyloxy)-1,4-phenylene}] (PFPV, MW=110 800), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV, MW = 680 000), and poly(9,9-dioctylfluorenyl-2,7-diyl) (PDOF, MW = 48 800). These conjugated polymers, the structures of which are shown in Figure 1, were obtained from American Dye Source, Inc. and used without further purification. Tetrahydrofuran (Ajax Finchem, Australia) was distilled before use to remove butylated hydroxyl toluene (peroxide inhibitor) and water. Dichloromethane and methanol (Merck, Australia) were used without any further purification. Water was obtained from a Millipore Milli-Q NANOpure water system including filtration with a 0.2 μm nylon membrane. Preparation of Conjugated Polymer Nanoparticles. Dispersions of CPdots were prepared according to the method described by Wu et al. with some minor alterations.5 The conjugated polymer was dissolved in freshly distilled tetrahydrofuran (THF) (9) Chandler, D. Nature 2005, 437, 640. (10) ten Wolde, P. R.; Chandler, D. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 6539. (11) Yang, Z.; Huck, W. T. S.; Clarke, S. M.; Tajbakhsh, A. R.; Terentjev, E. M. Nat. Mater. 2005, 4, 486.

Published on Web 11/11/2010

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Figure 1. AFM height images of conjugated polymer nanoparticles taken in situ for (a) MEH-PPV, (b) PDHF, (c) PDOF, and (d) PFPV. The corresponding histograms and chemical structures of the nanoparticle height are presented on the right. overnight at a concentration of 0.1 mg/mL in a nitrogen atmosphere and subsequently filtered through a 1.45 μm glass fiber filter (Whatman). The filtered solution was then diluted to a concentration of 20 ppm, and 2 mL of this resulting polymer solution was added to 8 mL of water under vigorous stirring. This step was repeated five times, and the resulting dispersions were combined. The water/THF dispersions were filtered through a 0.7 μm glass fiber filter (Whatman) and a 0.2 μm PTFE membrane filter (Sartorius Stedim Biotech) dampened with approximately 2 mL of either THF or methanol. The remaining THF was removed under reduced pressure, and the suspension was filtered again using methanol to dampen the PTFE membrane. Finally, the nanoparticle dispersion had a concentration of approximately 4 ppm. Care was taken to limit the conjugated polymers’ and CPdots’ ambient light exposure.

Fourier Transform Infrared, UV-vis Absorption, and Fluorescence Spectroscopy. Fourier transform infrared spectroscopic analysis was performed using a Perkin-Elmer Spectrum BX FT-IR system and a Nicolet Magna IR 750 spectrometer. 17786 DOI: 10.1021/la103063p

Clafton et al. The pristine polymer samples were cast directly from a 1 mg/mL polymer in dichloromethane (DCM) solution onto a NaCl plate or a ZnSe crystal. The aqueous nanoparticle dispersions, prepared as described above, were concentrated under reduced pressure to 20 ppm before being lyophilized overnight to remove the remaining water. Care was taken to avoid exposing the CPdot samples to light. The solid polymer was then dissolved in DCM, and a film was cast onto a NaCl plate or a ZnSe crystal. UV-vis spectra were collected on a Cary 5000 UV-vis-NIR spectrophotometer (Varian), and fluorescence spectra were recorded using a Cary Eclipse fluorescence spectrophotometer (Varian). The fluorescence quantum yields (Φf) of PDHF, PDOF, PFPV, and MEH-PPV CPdots were found to be approximately 28, 65, 4, and 3%, respectively. Compared to literature values,5,12 Φf of PDHF and PDOF are significantly higher. The discrepancies may be attributed to the different preparation conditions, which will be investigated in the future. Neat films of conjugated polymers and CPdots were prepared using the following methods. For the conjugated polymer films, PSS/PEDOT (Sigma-Aldrich) was filtered with a 0.2 μm Supor membrane filter and spin coated onto a glass slide. The conjugated polymer was then spin-coated from a 1 mg/mL solution in DCM. As for the CPdot films, a CPdot suspension was concentrated to approximately 40 ppm. The surface of a glass slide was covered with the suspension (around 1 mL) and left in a desiccator with dry ice under vacuum overnight. Fluorescence spectra of neat films were collected using 480, 380, 380, and 420 nm as excitation wavelengths for MEH-PPV, PDHF, PDOF, and PFPV, respectively. Zeta Potential Measurements. The Malvern ZetaSizer Nano S was used for zeta potential measurements. The prepared nanoparticle dispersions were concentrated to 40 ppm to achieve reliable results. All measurements were taken with 1 mM NaCl. All results were collected using either a low-volume disposable cuvette for size measurements or a disposable zeta potential cuvette at 25 °C. Instrument settings were automatically determined by Malvern dispersion technology software. Atomic Force Microscopy. The morphology and size distribution of the CPdots were characterized by atomic force microscopy (AFM) with a Nanoscope III (Digital Instruments, Santa Barbara, CA). Tapping-mode imaging in solution (in situ) was performed using a fluid cell and narrow, thin silicon nitride (Si3N4) cantilevers (V-shaped cantilever configuration) with a typical spring constant of 0.2 N/m and a resonance frequency of around 9 Hz. A piezoelectric tube scanner E with a 10  10 μm2 scan size in the XY direction and a 2.5 μm vertical range was used. The cantilever and tip were cleaned by immersion in ethanol, rinsing with copious amounts of high-quality Milli-Q water, and drying under high-purity nitrogen before use. All experiments were conducted in a class-100 clean room at 22 °C. The substrate used for the adsorption and imaging of the CPdots was a silicon wafer coated with titania (Philips Research Laboratories, Eindhoven, The Netherlands),13 which had been cleaned in piranha solution and ultrapure water. A pH 4 solution was chosen for the study to ensure that the wafer was slightly positively charged,13 thus facilitating the adsorption of the negatively charged CPdots. The cleaned wafer was mounted using C tape on a magnetic stainless steel disk. A commercially available liquid cell was used for these studies. One O-ring from the liquid cell was placed on top of the wafer, and the liquid cell was then lowered onto the O-ring to form a seal. The polymer solution was injected into the liquid cell using a 1 mL syringe. Wafers exposed to CPdot suspensions were imaged using the liquid cell, allowing for in situ time-resolved analysis for up to 15 min at room temperature. The in situ tapping-mode AFM images recorded after 15 min of immersion are shown in Figure 1. To facilitate comparison between images, all height images have been filtered in the same manner. The filtering option used was flattening (a second-order flattening process). (12) Wu, C.; McNeill, J. Langmuir 2008, 24, 5855. (13) Kanta, A.; Sedev, R.; Ralston, J. Langmuir 2005, 21, 2400.

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Figure 2. (a) Photographs of 4-week-old (left) and freshly prepared (right) PDHF CPdot suspensions under visible and UV light. (b) Zeta potential distribution of an aqueous PDHF CPdot suspension with a peak near -40 mV.

Elemental Analysis. The analysis was performed at the Campbell Micro Analytical Laboratory at the University of Otago, New Zealand. The samples were prepared using a 40 ppm polymer stock solution. The resulting nanoparticle suspensions were concentrated, lyophilized, dissolved in DCM, and dried in a 50 °C oven in an attempt to remove any water molecules that may be trapped inside the nanoparticles. X-ray Photoelectron Spectroscopy. Samples for XPS analysis were prepared as described above and concentrated to 40 ppm. XPS analysis was performed on a Kratos Axis Ultra DLD spectrometer using monochromated Al KR (1486.6 eV) X-rays operating at 130 W with a spot size of 700300 μm2. Measurements were taken with the sample surface normal to the detector and at a pressure of ∼5 10-9 Torr or lower. High-resolution C 1s spectra were acquired with a 0.1 eV step and a 390 ms dwell time, taking three sweeps per spectra. Charge neutralization was employed during analysis, and comparisons between spectra collected before and after analysis did not indicate any beam damage, such as peak shifting. Powdered samples were mounted directly onto the sample holder using conductive copper tape. Polymer films were drop cast onto silicon wafer substrates (that had been cleaned in piranha solution and ultrapure water) and allowed to dry in a desiccator for approximately 6 h. The silicon wafers were mounted on the XPS sample holder using conductive copper tape. Peak fitting was performed using CasaXPS (version 2.3). The peak-fitting function was a combination of 70% Gaussian and 30% Lorentzian (GL(30)) and was used to simulate a Voigt profile. The major aliphatic/aromatic C 1s peak was used as the internal calibration and assigned a binding-energy position of 285.0 eV. In some cases, the peak width of lower-intensity components (when the individual components were not fully resolved) was fixed to that of the most clearly defined C 1s component peak width.

Results and Discussions CPdots were prepared using a reprecipitation method with minor modifications.5 The CPdots produce bright fluorescence under UV illumination and show virtually no sign of aggregation or degradation 4 weeks after preparation (Figure 2, top panel). The height of the CPdots is approximately 10 nm (Figure 1) as determined by AFM, which is in good agreement with previous studies.3,5 The UV-vis absorption and fluorescence spectra of conjugated polymers and CPdots are presented in Supporting Information and show excellent agreement with previous results in that the spectra of CPdots exhibit a blue shift in absorption but Langmuir 2010, 26(23), 17785–17789

a red shift in fluorescence, relative to those of the corresponding polymers.5,6 As mentioned above, CPdots are present in an aqueous solution whereas the corresponding polymers are solubilized in an organic solvent (e.g., THF). To gain insight into the electronic transitions of CPdots and their corresponding polymers in the absence of solvent effects, neat films were prepared on glass substrates and characterized using UV-vis absorption and fluorescence spectroscopy (Supporting Information). First, the absence of a blue shift in the UV-vis absorption spectra of CPdot films relative to the corresponding conjugated polymer films suggests that the blue shift observed in solution is a solvent-induced effect. Second, all CPdot films, except PDOF, show a red-shifted fluorescence spectrum relative to that of the conjugated polymer film, suggesting that the structure and conformation of CPdots have an effect on the electronic transitions, which is consistent with the results of previous studies.14,15 Surface charge measurements were performed on CPdots to show a negative zeta potential. For instance, the zeta potential of the PDHF dot is approximately -40 mV (Figure 2, bottom panel), indicating a negatively charged surface. The zeta potential results for other CPdots are shown in Supporting Information. The values of the zeta potential for CPdots in the current study are consistent with previous measurements.8 Using the zeta potential results and the measured sizes of the CPdots, the surface charge densities can be calculated using an established model (Supporting Information).16 The surface charge density of the PDHF dot is (1.70 ( 0.17) 10-2 C/m2, which is comparable to those of stable colloidal systems.17 The surface charge densities of other CPdots have similar values and are included in the Supporting Information. In addition, zeta potential measurements were performed on CPdots at pH values ranging from 3 to 7. The results, which are shown in the Supporting Information, indicate that the zeta potential decreases in magnitude as the pH decreases. CPdot aggregation was observed within the range of pH 3 to 4, at which most CPdots have a zeta potential of -15 to -20 mV. Interestingly, no isoelectric point was detected in the experiment, which suggests that CPdots are present either in the anionic or neutral form but not in the cationic form within the pH range of this study. It is well known that stable polymer nanoparticles in water adopt a micellar structure consisting of a hydrophobic core and a hydrophilic shell.18,19 However, on the basis of their chemical structures, the conjugated polymers used in this study would exhibit strictly hydrophobic characteristics. Therefore, we hypothesize that chemical modification of the conjugated polymer during CPdot formation leads to hydrophilic defects. These defects in turn result in a negative zeta potential, maintaining long-term colloidal stability. Figure 3 shows the FTIR spectra of conjugated polymers (blue) and CPdots (red). Thin films of conjugated polymers and CPdots were prepared by drop casting CH2Cl2 solutions onto NaCl plates. Care was taken to remove water from the CPdot sample by lyophilization prior to thin film preparation. The conjugated polymer and CPdot FTIR spectra show several common peaks, including aliphatic and aromatic C-H stretches (2845-3072 cm-1) (14) Schwartz, B. J. Annu. Rev. Phys. Chem. 2003, 54, 141. (15) Hu, D. H.; Yu, J.; Padmanaban, G.; Ramakrishnan, S.; Barbara, P. F. Nano Lett. 2002, 2, 1121. (16) Elimelech, M.; Jia, X.; Gregory, J.; Williams, R. A. Particle Deposition and Aggregation: Measurement, Modelling, and Simulation; Colloid and Surface Engineering; Butterworth-Heinemann: Oxford, England, 1998. (17) Chen, G.; Abichou, T.; Tawfiq, K.; Subramaniam, P. K. Colloids Surf., A 2007, 302, 342. (18) Hawker, C. J.; Wooley, K. L. Science 2005, 309, 1200. (19) Zhang, Q.; Remsen, E. E.; Wooley, K. L. J. Am. Chem. Soc. 2000, 122, 3642.

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Figure 3. FT-IR spectra of conjugated polymers: (a) MEH-PPV, (b) PDHF, (c) PDOF, and (d) PFPV films of pristine polymer (blue) and films of CPdots (red). The highlighted peaks in the red spectra are evidence of the oxidation of conjugated polymers in the formation of CPdots. Although MEH-PPV, PDHF, and PDOF exhibit no IR peaks in the carbonyl region of the spectrum, the 1648 cm-1 peak of PFPV may be due to pre-oxidation (d, blue). Although the 1674 cm-1 peak of PFPV CPdot may be due to preoxidation, the other two peaks (d, red) are clearly evidence of polymer oxidation in the formation process.

and the methylene scissoring mode (1462 cm-1). However, three prominent peaks are present in the CPdot spectra but absent in those of conjugated polymers. For instance, in the MEH-PPV dot spectrum (Figure 3a), the peaks are assigned to a CdO induced aromatic stretch (1614 cm-1), an aromatic CdO stretch (1674 cm-1), and an aliphatic CdO stretch (1726 cm-1).20 Furthermore, these peaks are also present in the spectra of PDHF, PDOF, and PFPV dots, as shown in Figure 3b-d, respectively. The emergence of these peaks upon CPdot formation suggests the role of polymer oxidation in the reprecipitation process by which CPdots are prepared. X-ray photoelectron spectroscopy (XPS) was employed to provide additional insight into the role of polymer oxidation in the formation of CPdots. Figure 4 shows the XPS spectra of thin films of conjugated polymers (blue) and CPdots (red) in the C 1s region. The XPS spectra of MEH-PPV and PFPV (Figure 4a,d, blue) can be deconvoluted to show the presence of two peaks that are assigned to aliphatic/aromatic carbon (C1, 285.0 eV) and C-O (C2, 286.2 eV). The presence of these peaks shows strong agreement with the chemical structures of MEH-PPV and PFPV. In contrast, the XPS spectra of MEH-PPV and PFPV CPdots (Figure 4a,d, red) exhibit additional peaks at higher binding energies. (20) Stuart, B. H. Infrared Spectroscopy: Fundamentals and Applications; John Wiley & Sons: Chichester, U.K., 2004.

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Figure 4. C 1s XPS spectra and deconvoluted peaks of (a) MEHPPV, (b) PDHF, (c) PDOF, and (d) PFPV films of pristine polymer (blue) and films of CPdots (red). Evidence for the oxidation of conjugated polymers in the formation of CPdots is shown by either a relatively higher intensity of the C2 signal (a, d) or the emergence of this signal (b, c). In addition, the appearance of the C3 (all except the PFPV CPdots) and C4 (all CPdots) peaks provides additional evidence. These results support the FT-IR results in Figure 3.

Deconvolution of these spectra shows evidence of polymer oxidation due to the presence of two additional peaks: CdO (C3, 287.9 eV) and O-CdO (C4, 289.4 eV). Besides, the amplitudes of the C2 peak of MEH-PPV and PFPV CPdots are higher than those of the corresponding polymers. It is important to note that the peak positions in this study are in good agreement with the results in an earlier study.21 Furthermore, the XPS spectra of PDHF and PDOF are straightforward because only one peak is present, which is clearly assigned to C1, as expected on the basis of the chemical structures of these polymers (Figure 4b,c, blue). However, new peaks at higher binding energies emerge in the XPS spectra of PDHF and PDOF CPdots, which are assigned to C2, C3, and C4 using spectral deconvolution (Figure 4b,c, red). The presence of these peaks is unequivocal evidence that oxidation is involved in the formation of CPdots. The details of the deconvoluted peaks including peak position, width, and area are summarized in Table 1. In short, the XPS results are consistent with the (21) Briggs, D.; Beamson, G. Anal. Chem. 1992, 64, 1729.

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Table 1. XPS Fitting Parameters for Raw Polymer and CPdot Samples as Determined Using CasaXPSa

Table 2. Elemental Composition of Pristine Conjugated Polymer Samples and CPdots in Mole %a

C1 pos/ C2 pos/ C3 pos/ C4 pos/ fwhm/% area fwhm/% area fwhm/% area fwhm/% area

C

H

38.91 (39.53) 43.32 (43.86) 42.05 (42.03) 40.37 (41.38)

56.46 (55.81) 56.68 (56.14) 58.03 (57.97) 57.05 (56.90)

N

O

Polymerb Polymer MEH-PPV PDHF PDOF PFPV

285.0/1.0/60 285.0/0.9/100 285.0/1.0/100 285.0/1.0/73

286.2/1.7/40

MEH-PPV PDHF PDOF PFPV

286.2/2.2/26

CPdot

285.0/1.2/31 285.0/1.3/54 285.0/1.3/83 285.0/1.1/63

286.1/1.4/56 286.0/1.3/28 286.2/1.3/8 286.6/1.4/25

CPdot MEH-PPV PDHF PDOF PFPV

287.9/1.4/8 287.3/1.3/7 287.4/1/3/5

289.4/1.4/5 288.7/1.3/11 289.1/1.3/4 288.7/1.5/12

a The peak position (pos) and fwhm are in eV; the peak area is in percentage.

FTIR results, strongly supporting the oxidation of polymers in the formation of CPdots. Although FTIR and XPS are crucial for detecting the presence of chemical defects of CPdots, these spectroscopic techniques do not allow one to determine the level of oxidation of CPdots accurately and quantitatively. In particular, XPS is surface-sensitive and hence can provide chemical analysis only on the surface level. Therefore, to determine the level of oxidation, quantitative elemental analysis was performed on the conjugated polymers and CPdots. Results on the individual conjugated polymers show a high degree of agreement with the expected values from the chemical structure of the polymers, as shown in Table 2. However, results on the CPdots indicate that the level of oxygen originating from the formation of nanoparticles has a range of 7-13 mol % (Table 2). These results not only further support the FTIR and XPS results but also provide crucial insight into the quantitative level of oxidation in CPdot formation for the first time. We have considered the possibility of polymer oxidation in THF. Although THF is known to produce peroxides over time, FT-IR experiments performed on the polymer-THF solutions over 24 h showed no evidence of polymer oxidation. This finding is consistent with the relatively long timescale of peroxide generation as shown by the results from another study.22 However, although precautions were taken to reduce oxidation during lyophilization, because of the low polymer concentration in the CPdot solutions, even a small amount of THF peroxides or other oxidants could be greatly concentrated during this process, which may accelerate oxidation. The effect of the photo-oxidation of CPdots by ambient light was also investigated, but the results, (22) http://www.ehs.washington.edu/forms/epo/peroxideguidelines.pdf.

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4.69 (4.65) 0.01 (-) -0.08 (-) 2.58 (1.72)

MEH-PPV 34.28 52.68 0.39 12.64 PDHF 37.30 48.84 0.41 13.45 PDOF 38.48 54.45 0.22 6.84 PFPV 35.79 51.96 0.36 11.88 a Strong agreement between analysis runs was confirmed using duplicate measurements. The error in the elemental analysis technique is approximately 0.21%. However, the error in the oxygen percentage is likely to be larger as a result of variations in the CPdot preparation process. b The values in parentheses are calculated elemental compositions from the known monomeric structures of the conjugated polymers.

which are shown in the Supporting Information, indicate that the level of oxidation by this mechanism is negligible. In short, although we cannot rule out the possibility that some oxidation occurs during lyophilization, our future work will address this issue by focusing on the origin and mechanism of oxidation involved in CPdot formation.

Conclusions We report the presence of chemical defects in highly fluorescent CPdots. These defects are necessary to enable the otherwise hydrophobic nanoparticles to form an aqueous suspension with colloidal stability. The results of this work offer new insights into the interactions of conjugated hydrophobic species with water. Acknowledgment. This work was supported by the Australian Research Council/National Health and Medical Research Council Network FABLS. We acknowledge Jason D. McNeill for insightful discussions and a critical reading of the manuscript. We dedicate this manuscript to the late Prof. Paul F. Barbara who encouraged T.W.K. to conduct this work. Supporting Information Available: UV-vis absorption and photoluminescence spectra, results of zeta potential measurements, surface charge density calculations, the zeta potential as a function of pH, and UV-vis absorption spectra of PDOF CPdots as a function of ambient light exposure. This material is available free of charge via the Internet at http://pubs.acs.org.

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