Supplementary Material (ESI) for Chemical Communications This journal is (c) The Royal Society of Chemistry 2009 1 Supporting Information on pH Sensor Based on Upconverting Luminescent Lanthanide Nanorods Li-Ning Sun, Hongshang Peng, Matthias I. J. Stich, Daniela Achatz and Otto S. Wolfbeis* Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg, D-93040 Regensburg, Germany Materials and buffers Y(NO 3 ) 3 , Yb(NO 3 ) 3 , Er(NO 3 ) 3 , oleic acid, and Bromothymol Blue (BTB) were obtained from Aldrich. The polyurethane hydrogel D4 was obtained from Cardiotech. The polyester support made from poly(ethylene terephthalate), with a thickness of 125 μm, was from Goodfellow (Cambridge, UK). Doubly distilled water was used for the preparation of the buffer solutions. Their pH was determined with a digital pH meter (Schott Instruments) calibrated at 20 ± 2 °C with standard buffers of pH 7.0 and 4.0 (Merck). Synthesis of the NaYF 4 :Er,Yb nanorods NaOH (1.2 g, 30 mmol), water (5 mL), ethanol (7 mL), and oleic acid (20 mL) were mixed under agitation to form a homogeneous solution. Then Y(NO 3 ) 3 (950 μL of a 0.5 M solution), Yb(NO 3 ) 3 (75 μL of a 0.2 M solution) and Er(NO 3 ) 3 (50 μL of a 0.2 M solution) were added under magnetic stirring. Subsequently, 8 mL of a 1.0 M aqueous solution of NaF were added dropwise. The mixture was stirred for about 20 min, then transferred to a 50 mL autoclave, sealed, and hydrothermally treated at 190 °C for 24 h. The system was left to cool to room-temperature. Cyclohexane (about 15 mL) was used to dissolve and collect the products by sonication. The products were subsequently deposited by adding ethanol to the cyclohexane solution. Following centrifugation at 8000 rpm, the powders obtained were purified by several washing/centrifugation cycles (ethanol) to remove oleic acid, sodium oleate, and other remnants. TEM measurements show some nanoparticles also being formed along with the nanorods. Experimental results indicated that most of the nanoparticles can be removed by centrifuging and washing. Preparation of sensor membrane The indicator BTB (2 mg) was dissolved in 1.6 g of a solution of 1.0 g of the polyurethane hydrogel D4 in 8.1 g of 99 % ethanol and 0.9 g of water and by vigorously stirring it for 5 h. Then, 20 mg of the NaYF 4 :Er,Yb nanorods were added and the solution stirred for 10 h. The mixture (the "cocktail") was spread onto the polyester support using a home-made device. The resulting layer was dried at room temperature. From the quantities of materials and solvents employed, the thickness of the dried membrane was calculated to be 12 µm. A cross section of the sensor film is given in Fig. S1. Fig. S1. Cross section of the hydrogel-based film used for sensing pH. The thickness of the upper (sensing) film is ~ 12 µm, that of the polyester support 125 µm.