Supplementary Information Dynamically tunable nanoparticle engineering enabled by short contact-time microfluidic synthesis with a reactive gas Md. Taifur Rahman , Prasanna G. Krishnamurthy , Pravien Parthiban , Abhinav Jain , Chan Pil Park , Dong-Pyo Kim and Saif A. Khan Experimental Details Materials: 3-aminopropyl tris(trimethylsiloxy)silane (APTTS, 99%), hydrogen tetrachloroaurate(III) trihydrate (HAuCl4, 99.99%), tetrakis(hydroxymethyl) phosphonium chloride (THPC, 80% in water), sodium hydroxide (NaOH, Reagent grade, 97%), acetonitrile (CH3CN, HPLC grade 99.9%), potassium carbonate (K2CO3, 99.99%), Octadecafluorodecahydronaphthalene (Perfluorodecalin, Fluka, mixture of cis and trans, 95%) from Sigma-Aldrich Co. Ltd., Singapore, were all used as obtained without any further purification. De-ionized water and glassware washed in aqua regia and rinsed thoroughly in water were used for all experiments. Synthesis of amino-functionalized silica spheres: 230 nm silica particles were synthesized by the Stöber method. 1 The surfaces of silica spheres were functionalized with APTTS in refluxing acetonitrile. Amino-functionalized silica particles were washed with acetonitrile, ethanol and DI water by sonication and several cycles of centrifugation. 2,3 Gold-seeded silica particles: 2-3 nm colloidal gold particles used for seeding the amino- functionalized silica was made according to the procedure of Duff et al. 4 Under rapid stirring 1.5 mL of 0.2 mM aqueous NaOH and 1.0 mL of 67.7 mM aqueous THPC were sequentially added to 45.5 mL of DI water followed by addition of 2 mL of aqueous 25 mM HAuCl 4 . The solution immediately turned brown-red indicative of the colloidal gold formation. 150 µL of amino-functionalized silica particles suspension with 20ml of colloidal gold solution was shaken for 8 hours at room temperature and centrifuged to separate the gold-seeded silica particles. These particles were washed three times with water by cycles of centrifugation to remove the unattached colloidal gold particles. K-Gold Solution: 3 mL of 1 wt% HAuCl4 was taken with 50 mg K 2 CO 3 in 200 mL DI water (1.8mM) and aged for 1 day to produce a colorless gold plating solution, so-called K- gold, containing gold hydroxide ions. 5 Microfabrication: Microfluidic device patterns were fabricated onto silicon wafers by standard photolithography using negative photoresist SU-8 2050. 6 Devices were subsequently moulded in poly(dimethyl siloxane) (PDMS) using the soft lithography technique. PDMS was molded onto the SU-8 masters at 70˚C for 2 hrs, peeled, cut and cleaned. Inlet and outlet holes (1/16-in. o.d.) were punched into the device. The microchannels were irreversibly bonded to a glass slide pre-coated with a thin layer of PDMS after a brief 35 s air plasma treatment. The bonded devices were then cured for 24 hours at 100˚C. The microchannels have rectangular cross-section and the details of the channel dimensions are: width and depth of the liquid and gas channel are 300 μm and ~124 μm and 100 μm and ~124 μm respectively, and the parallel liquid and gas channels are separated by 200 μm. Devices with lengths of the parallel channel section of 91.5 cm and 21.8 cm were used. Electronic Supplementary Material (ESI) for RSC Advances This journal is © The Royal Society of Chemistry 2013