S1 In Situ Observation of Place Exchange Reactions of Gold Nanoparticles. Correlation of Monolayer Structure and Stability Rui Hong, † Joseph M. Fernández, † Hiroshi Nakade, † Rochelle Arvizo, † Todd Emrick ‡ and Vincent M. Rotello † * , Department of Chemistry † , Department of Polymer Science and Engineering ‡ , University of Massachusetts, Amherst, Massachusetts, 01003 Supporting Information Materials: Thiols used in the place exchange reactions were purchased from Aldrich, and were used as received. (S)-2-Methybutyl-4-toluenesulfonate was synthesized according to a reported method. 1 The thiol-Bodipy ligand was synthesized as reported. 2 HAuCl 4 ·xH 2 O was purchased from Strem. Other chemicals were obtained from either Aldrich or Acros, and were used as received. Dichloromethane (DCM) was distilled over CaH 2 prior to use. Instrumentation: Fluorescence spectra of Bodipy release were collected on a Shimadzu RF-5301 PC spectrofluorometer. UV-Vis spectra were recorded on a HP 8452A UV-spectrometer. Proton NMR spectra were recorded using a Brüker DPX-300 spectrometer in CDCl 3 solutions at 300 MHz (referenced to residual CHCl 3 ). Thermogravimetric analysis (TGA) was performed using a TA Instruments, Inc. (New Castle, DE), Model TGA 2950 high-resolution thermogravimetric analyzer. The instrument was equipped with an open platinum pan and an automatically programmed temperature controller. The TGA curves were measured as follows: about 5 mg of each nanoparticle was placed in the TGA pan and heated in a nitrogen atmosphere at a rate of 10°C/min up to 450°C. Synthesis of (S)-2-methyl-1-butanlthiol (S)-2-methyl-1-butanthioacetate To a suspended solution of potassium thioacetate (3.4 g, 30 mmol) in dry THF (40 mL) was added (S)-2-Methybutyl-4-toluenesulfonate (2.4 g, 10 mmol) at room temperature, subsequently refluxed overnight. Water was added to quench the reaction, and the mixture was extracted with ether. Combined organic layer was washed with water and brine before dried over MgSO 4 . Concentrated residue was subjected to purification by flash column chromatography and distillation under reduced pressure to yield a colorless liquid (1.0 g, 68%) (S)-2-methyl-1-butanlthiol To a solution of (S)-2-methylbutylthioacetate (1.0 g, 6.8 mmol) in methanol (1 mL) was added oxalyl chloride (0.6 mL) dropwise. After stirring at room temperature over night, the solvent was distilled off at 120 ˚C. Crude product was purified by vacuum distillation