Supporting Information

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1 Supporting Information Electrogenerated Chemiluminescence of Single Conjugated Polymer Nanoparticles Ya-Lan Chang, Rodrigo E. Palacios, Fu-Ren F. Fan, Allen J. Bard, and Paul F. Barbara Department of Chemistry and Biochemistry, Center for Nano and Molecular Science and Technology and Center for Electrochemistry, the University of Texas at Austin, Austin, TX 78712, USA Correspondence should be addressed to PFB Experimental Section Materials. F8BT (MW=70 kg/mol) was purchased from American Dye Source (ADS). Acetonitrile, tetrahydrofuran, cyclohexane were Certified Grade anhydrous (Achro Seal, water < 50 ppm) and were used without further purification. Lithium perchlorate (LiClO 4 ), and tri-n-propylamine (TPrAH) were purchased from Sigma- Aldrich. ITO coated cover slips were purchased from Metavac. Custom-made silicone spacers were purchased from Grace Bio-Labs, Inc. Silver wire (25 µm) was purchased from Goodfellow. Silver paint was purchased from Acme Chemicals and Insulation Co. Preparation Procedures of Nanoparticles, Samples and SMS-EC Cells. Nanoparticles (NPs) were prepared by a reprecipitation technique. In brief, 12 µl of a tetrahydrofuran solution of F8BT (O.D. at 458nm = ~ 4.8) is flash -injected into 2 ml of cyclohexane. The size of NPs (r = 25 ± 15 nm) was estimated by comparing the single NP fluorescence intensity distribution to that of a reference F8BT NP sample with a known size distribution (measured by Scanning Electron Microscopy). An aliquot of the resulting colloidal suspension is spin-cast at 2,000 r.p.m. to deposit the F8BT NPs on the ITO electrode. The NPs adhere well to the ITO electrode (i.e. they are not washed away by the electrolyte solution), thus a host polymer layer (e.g. Nafion, polystyrene) 1 was not used in these experiments. SMS-EC cells used in these experiments comprised 3 electrodes in which Ag wire (QRE) was inserted in the gap between a planar transparent ITO (WE) and a planar Au (CE). Potentials relative to a ferrocenium/ferrocene (Fc/Fc+) couple internal standard (introduced into the cell after the SMS-EC data were acquired) are about 0.20 ( 0.04 V more negative than those of QRE. The gap between the planar electrodes was filled with an acetonitrile solution containing 0.1 M supporting electrolyte, 1

2 LiClO 4 and 0.1 M coreactant, TPrAH. The time dependence of the luminescence intensity of individual spots, each due to a single F8BT nanoparticles, was determined from a set of images that were recorded by a wide-field microscope (Eclipse TE2000, Nikon ; Roper Scientific, Cascade 512B, CCD camera) using an image analysis software home-written in Matlab. A potentiostat (PGSTAT 100, Autolab ) was employed to apply triangular or step potentials on the SMS-EC cells and simultaneously collect current profiles. The acquisition of images was synchronized to the application of potentials. ECL formation mechanism of the F8BT/TPrAH System F8BT is represented as R. Oxidation reaction of F8BT R e R Hole Transport (conduction) R (double layer at electrode) R (surface of NP) Oxidation reaction of TPrAH followed by a sequential deprotonation reaction Pr 2 NCH 2 CH 2 CH 3 e Pr 2 NCH 2 CH 2 CH 3 Pr 2 NCH 2 CH 2 CH 3 Pr 2 NC HCH 2 CH 3 + H + Neutralization of polymer radical cations (R + ) by neutral TPrAH and TPrA Pr 2 NCH 2 CH 2 CH 3 + R Pr 2 NCH 2 CH 2 CH 3 + R Pr 2 NC HCH 2 CH 3 + R Pr 2 NC + HCH 2 CH 3 + R ECL could be formed by two schemes: Scheme 1. Excited state of polymer is formed via electron transfer from TPrA radical (TPrA ) to R + R + Pr 2 NC HCH 2 CH 3 R* + Pr 2 NC + HCH 2 CH 3 R* R + h Scheme 2. First, neutral R is reduced by TPrA by which polymer radical anions (R - ) are formed. R + Pr 2 NC HCH 2 CH 3 R - + Pr 2 NC + HCH 2 CH 3 Second, excited state of polymer (R*) is formed by annihilation of R and R - R + R - R* + R R* R + h 2

3 Cyclic voltammogram of F8BT thin film on ITO Figure S1. (a) Voltammograms of F8BT thin film (25 nm) on ITO acquired at a scan rate of 0.1 V/s. The insert shows a magnified view of the current dashed box region. (b) Normalized ensemble fluorescence of F8BT films corresponding to voltammograms shown in a. In these experiments the illuminated area is less than 0.1 mm 2 whereas the measured current corresponds to an electrode area of ~ 40 mm 2. Consequently the measured current is not significantly affected by the laser excitation. Oxygen effect on ECL rise time Molecular oxygen is known for quenching the luminescence in several ECL systems 2-5, thus the observed ECL build-up time could in principle be related to slow depletion of oxygen in our cells during ECL production. In order to test this hypothesis we prepared samples and carried out experiments in a controlled environment. Samples were prepared inside a glove-box where oxygen and water concentrations were less than 10 and 1 ppm respectively. Anhydrous solvents were open inside the glove box and stirred for > 48 hrs in open containers prior to use. After preparation, the samples were transferred in airtight vessels to a box continuously over-filled by a flux of ultra pure argon (Matheson Tri-Gas) which was constructed over the microscope stage. The results of these experiments (see Fig S2) are qualitatively similar to those from experiments carried out in ambient 3

4 conditions, suggesting that water and oxygen do not play a significant role in the observed ECL build-up phenomena. However, a better degassing and drying of the solvents as well as a better control of the environment during the experiment are necessary to definitively address the effect of water and oxygen on the ECL build-up phenomena. Figure S2. (a) Applied bias. (b) Ensemble average of 670 single NPs ECL intensity trajectories measured in a pristine cell under ambient conditions while applying the bias show in a (c) Same as b (280 single NPs) measured in oxygen depleted conditions (see text for details). 4

5 Correlation between PL and ECL from single particles. Figure S3. Correlation between ECL and PL intensity of individual nanoparticles. Data corresponding to Fig. 3 were used to construct the scatter plot, 69 NPs emission intensity transients sowing both ECL and PL were analyzed. The reported ECL intensities were obtained integrating the measured counts in each transient from 6 s to 40 s (see transients ensemble in Fig. 3b). The reported PL intensities were integrated for 3 s at 0 V before the applied 1.8V (raw data not shown). ECL of bulk F8BT thin films Supporting the need for nano resolved ECL methods, preliminary experiments on ECL of bulk F8BT thin films collected with the same apparatus used in the SMS experiments reveal highly spatially and kinetically heterogeneous ECL behavior. These effects were assigned to cracks, pin-holes, variation due to film thickness and complex break-in processes on a range of distances and time scales. Such effects were missed in previous ECL studies on bulk films due, apparently, to the lack of spatial resolution. References (1) (a) Palacios, R. E.; Fan, F.-R. F.; Bard, A. J.; Barbara, P. F. J. Am. Chem. Soc. 2006, 128, (b) Palacios, R. E.; Fan, F.-R. F.; Grey, J. K.; Suk, J.; Bard, A. J.; Barbara, P. F. Nat. Mater. 2007, 6, 680. (2) Dennany, L.; Hogan, C. F.; Keyes, T. E.; Forster, R. J. Anal. Chem. 2006, 78, (3) Wallace, W. L.; Bard, A. J. J. Phys. Chem. 1979, 83, (4) Wallace, W. L.; Bard, A. J. J. Electrochem. Soc. 1978, 125, (5) Kim, J.; Faulkner, L. R. J. Electroanal. Chem. 1988, 242,