Supporting Information. Poly(ethylene glycol) Ligands for High-Resolution Nanoparticle Mass Spectrometry

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1 Supporting Information Poly(ethylene glycol) Ligands for High-Resolution Nanoparticle Mass Spectrometry Joseph B. Tracy, Gregory Kalyuzhny, Matthew C. Crowe, Ramjee Balasubramanian, Jai-Pil Choi, and Royce W. Murray*, Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina Department of Chemistry and Biochemistry, San Diego State University, San Diego, California Preparation and Purification of Gold Nanoparticles: A solution of 3.1g HAuCl 4 3H 2 O in ml deionized water was mixed with 5. g tetraoctylammonium bromide (TOABr) in ml toluene. After stirring for 3 minutes to complete the phase transfer of the gold precursor into the toluene phase, the toluene phase was separated from the water, and 3.43 ml phenylethanethiol (HSC2Ph) was added to the toluene solution and stirred overnight to form Au(I)-thiolate complexes. The next morning, the toluene solution was cooled in an ice bath, and 3.8 g NaBH 4 dissolved in 6 ml ice-cold water was quickly added while stirring rapidly with a mechanical stirrer. The temperature of the ice bath was maintained at C for 24 hours with vigorous stirring. Using a separatory funnel, the aqueous layer was discarded, and ml water was added in order to remove excess NaBH 4 and reaction side products. After shaking to mix the layers, the aqueous layer was again discarded. This procedure of adding ml water, shaking, and discarding the aqueous layer was repeated two more times. The toluene was removed at room temperature on a rotary evaporator. An oily residue remained, to which ml of proof ethanol was added. After 2 hours, most of the product had precipitated, and the precipitate was collected on a glass frit (fine porosity). The precipitate was transferred to a new flask by rinsing the frit with dichloromethane and then evaporating the dichloromethane. The sample was then treated with methanol for 45 minutes in order to remove excess TOABr and HSC2Ph, after which the methanol was discarded, and the sample was dried. The product was extracted by adding acetonitrile and then removing the acetonitrile. Multiple extractions with acetonitrile were generally necessary, and if the solid product still had an oily appearance, additional methanol treatments were performed. The methanol treatment makes the acetonitrile extractions more efficient. The product purity was verified with UV-Vis absorbance and voltammetry measurements. This product has been reported previously as Au 38 (SC2Ph) 24 (Ref. 7c), but it is shown in this paper to be Au 25 (SC2Ph) 18. S-1

2 The 4+ peaks are consistent with the assignment of M 5 Au 25 (ligand) : (a) x x (b) Figure S-1: (a) Set of 4+ peaks acquired using.44 mg (green) and.89 mg (red; scaled by 1.625) NaOAc per mg NPs, after 24 hours of exchange. (b) High resolution analysis of the same data compared with simulations (black) of x 4+ = Na 5 Au 25 (SC2Ph) 18-x (S-PEG) x 4+. S-2

3 Detailed justification for assignment Au 25 (ligand) 18 : The possible assignments for the peak that is centered at m Da [Fig. 1(c), middle panel] are shown in Table S-1. Average masses outside the range given there and generally outside a window of ±1 Da from the center of the experimental peak do not match the data. Resolution alone is not sufficient to eliminate the other possibilities, but the assignment of Na 4 Au 25 (SC2Ph) 9 (S-PEG) 9 matches the data well. Figure 2 moreover, shows that the species with m Da has undergone at least 9 successive replacements of -SC2Ph with -S-PEG. Because at least 12 successive ligand exchanges are observable in Figure 2, the Da species must have at least 9 -S-PEG and 3 -SC2Ph ligands. Therefore, all of the possible assignments in Table S-1 may be eliminated except for Na 4 Au 16 (SC2Ph) 18 (S-PEG) 11, Na 4 Au 25 (SC2Ph) 9 (S-PEG) 9, and Na 4 Au 21 (SC2Ph) 5 (S-PEG) 14. In order to cement a unique assignment, we consider the peak generated by the unexchanged material with m 7463 Da. There are many fewer possible assignments, because this species has no -S-PEG. Assignments near this mass are listed in Table S-2, where Na 3 Au 25 (SC2Ph) is the only match with the data and is therefore the unique assignment. High resolution data and simulations of Au 25 (ligand) 18 for each extent of ligand exchange are shown in Fig. S-2. In particular, we note that the match for 2+ is excellent. Because 2+ is a unique assignment, the other x n+ = Na n+1 Au 25 (SC2Ph) 18-x (S-PEG) n+ x that are generated by successive ligand exchange are also unique assignments. S-3

4 Table S-1: All possible assignments for the peak assigned as Na 4 Au 25 (SC2Ph) 9 (S-PEG) 9 with masses in a window greater than ± 1 Da for 15 n Au 4, n SC2Ph 25, and n S-PEG 25. Average mass (Da) Assignment Na 4 Au 16 (SC2Ph) 18 (S-PEG) Na 4 Au 4 (SC2Ph) Na 4 Au 36 (SC2Ph)(S-PEG) Na 4 Au 29 (SC2Ph) 13 (S-PEG) Na 4 Au 25 (SC2Ph) 9 (S-PEG) Na 4 Au 21 (SC2Ph) 5 (S-PEG) Na 4 Au 17 (SC2Ph)(S-PEG) Na 4 Au 22 (SC2Ph) 25 (S-PEG) Na 4 Au 18 (SC2Ph) 21 (S-PEG) Na 4 Au 38 (SC2Ph) 4 (S-PEG) 2 S-4

5 Table S-2: All possible assignments for the peak assigned as Na 3 Au 25 (SC2Ph) with masses in a window greater than ± 5 Da for 15 n Au 4 and n SC2Ph 25. Average mass (Da) Assignment Na 3 Au 34 (SC2Ph) Na 3 Au 25 (SC2Ph) Na 3 Au 32 (SC2Ph) 8 S-5

6 High resolution analysis of different extents of ligand exchange: In Fig. S-2, high resolution data for the most intense peaks for different extents of ligand exchange are compared with simulations of the assigned species, x n+ = Na n+1 Au 25 (SC2Ph) 18-x (S- PEG) n+ x. Because single isotopes are not consistently resolved, the match is assessed by how well the simulated curve is centered within the experimental data. The matches between the data and simulations are excellent. Atomic resolution has been achieved; if the simulated mass is increased or decreased by 1 Da, then the simulation will no longer match with the data. For each major peak, n+1 Na + ions and a total ion charge of n+ leaves a 1- charge on the core. High resolution analysis of the minor peaks in Fig. 2 (labeled a and b) and comparison with simulations is presented in Fig. S-3. The assignments for different extents of ligand exchange are given in Table S-3. Initially, we thought that these minor peaks were generated by protonation rather than Na + coordination, which would preserve the 1- core charge indicated by the major peaks. The less intense signals in Fig. S-3 reduce the resolution as compared with Fig. S-2, but the Fig. S-3 high resolution analysis supports the idea that 1b 2+, 1a 2+, 2b 2+, and 2a 2+ have lost Na + without coordination to protons, thereby leading to more positive ( or 1+) core charges. The 3a 2+ ion, however, does appear to be protonated. Further investigation is underway to more persuasively demonstrate these effects and to understand the origins of these minor peaks. S-6

7 hours, 4 hours, 4 2 hours, 2+ 2 no exchange, Figure S-2: High resolution analysis of different extents of ligand exchange from Fig. 2. Simulations (thick black curve) and assignments are x n+ = Na n+1 Au 25 (SC2Ph) 18-x (S-PEG) x n+. S-7

8 1b 2+ 1a 2+ 2b a 2+ 3a hours, hours, 2 hours, 2+ no exchange, 2+ Figure S-3: High resolution analysis and simulations (thick black curves) of minor peaks that are identified by labels a and b in Fig. 2. Assignments are given in Table S-3. S-8

9 Table S-3: Assignments for peaks in Fig. S-3. Label Assignment Core charge 1b NaAu 25 (SC2Ph) 17 (S-PEG) a Na 2 Au 25 (SC2Ph) 17 (S-PEG) 1 2b NaAu 25 (SC2Ph) 16 (S-PEG) a Na 2 Au 25 (SC2Ph) 16 (S-PEG) 2 3a HNa 2 Au 25 (SC2Ph) 15 (S-PEG) 3 1- S-9

10 Optical spectra and voltammetry before and after ligand exchange: The data for different extents of ligand exchange indicate that no change in core size occurs during ligand exchange, and optical spectra and electrochemistry also exhibit no substantial difference between the un-exchanged material and the exchange product. If a change in core size had occurred, the electronic structure of the nanoparticles might be substantially altered, which would be reflected in optical and electrochemical measurements. Absorbance and luminescence spectra (Fig. S-4) indicate no substantial changes. In particular, we believe that a change in core size should alter the absorbance band at ~1.8 ev. While the voltammetric features (Fig. S-5) have changed slightly, this can be attributed to the differences in the electronic coupling of -SC2Ph and -S-PEG to the Au core and to the different dielectric properties of the ligand monolayers. A change in core size would probably lead to more significant changes in the electrochemistry. S-1

11 Abs., normalized at 1.8 ev (arbitrary units) Abs. (arbitrary units) hours 4 hours 1 hour before exchange E (ev) Photoluminescence normalized to absorbance max(~1.8 ev) (arbitrary units) E (ev) Figure S-4: Absorbance and photoluminescence (45 nm excitation) measurements of Au NP samples in dichloromethane before and after different times of ligand exchange. Inset: the same absorbance spectra expanded near the absorbance edge. S-11

12 I (μa) I (μa) CV DPV V (mv) vs. Ag/AgCl 22.5 hours before exchange - - Figure S-5: (a) Cyclic voltammetry (CV) and (b) differential pulse voltammetry (DPV) of Au NP samples before and after 22.5 hours of ligand exchange with HS-PEG. Measurements were taken in a 1:1 (vol) mixture of acetonitrile and toluene with.1 M tetrabutylammonium perchlorate using.5 mm Pt working and Ag/AgCl reference electrodes. S-12