Comparison of Water-Soluble CdTe Nanoparticles Synthesized in Air and in Nitrogen Yuanfang Liu, ² Wei Chen,* ,²,‡ Alan G. Joly, § Yuqing Wang, | Carey Pope, ⊥ Yongbin Zhang, ⊥ Jan-Olov Bovin, # and Peter Sherwood | Nomadics, Inc., 1024 South InnoVation Way, Stillwater, Oklahoma 74074, Department of Physics, The UniVersity of Texas at Arlington, Arlington, Texas 76019, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, Department of Physics, Oklahoma State UniVersity, Stillwater, Oklahoma 74078, Department of Physiological Sciences, Oklahoma State UniVersity, Stillwater, Oklahoma 74078, and Materials Chemistry, Lund UniVersity, P.O. Box 124, SE-22100, Lund, Sweden ReceiVed: May 19, 2006; In Final Form: July 5, 2006 It is commonly believed that high-quality CdTe nanoparticles with strong luminescence can only be prepared under the protection of an inert gas such as nitrogen or argon. Here, we report the preparation of highly luminescent CdTe nnaoparticles in air and compare their luminescence properties with CdTe nanoparticles made in nitrogen. We find that both water-soluble CdTe nanoparticles made in air and in nitrogen exhibit strong photoluminescence as well as upconversion luminescence at room temperature. However, differences do exist between the particles made in air and those made in nitrogen. In particular, the particles prepared in air display a faster growth rate, grow to larger sizes, and display stronger electron coupling relative to the particles prepared in nitrogen. X-ray photoelectron spectroscopy analysis indicates that the oxygen content in the nanoparticles synthesized in air is higher that that in particles synthesized in N 2 , likely resulting in a higher availability of excess free cadmium. Cytotoxicology measurements reveal that the particles made in air appear slightly more toxic, possibly due to the excess of free cadmium. 1. Introduction CdTe nanoparticles have been the subject of numerous investigations. Because of high quantum efficiency and multi- color availability, CdTe nanoparticles can find applications in solid-state lighting, displays, optical communications, sensors, as well as in biological imaging and detection. 1 Currently, two synthesis strategies, nonaqueous synthesis and aqueous synthe- sis, are used to prepare CdTe nanoparticles. As compared to the nonaqueous synthesis, aqueous synthesis is more reproduc- ible, cheaper, less toxic, and the “as-prepared” samples are more water-soluble and bio-compatible. 2 Water-soluble semiconductor nanoparticles can be obtained mainly by two different methods. The first way is to replace the surface-capping molecules on the particles prepared by the TOPO (trioctylphosphine oxide) method with water-soluble thiols or a silica shell. 3-6 However, after the substitution of the surface-capping molecules by hydrophilic molecules, the nano- particle photoluminescence decreases markedly. 3,7,8 The second method is to directly synthesize semiconductor nanoparticles in aqueous solution using water-soluble stabilizers such as thiols. 2,8,9 The second method has become a popular recipe for making water-soluble nanoparticles. It is generally believed that the preparation should be conducted in inert or reduced atmospheres because oxygen in the air can oxidize the nano- particles and quench the luminescence. However, high-quality CdSe quantum dots have been prepared using organic solvents both in air and in an inert atmosphere. 10 In our work, we find that highly luminescent water-soluble CdTe nanoparticles can be synthesized in air and their luminescence efficiency is comparable to or higher than that of particles made in nitrogen. In this Article, we compare the structural, optical, and cytotoxic properties of CdTe nanoparticles made under similar conditions but using either nitrogen or air atmospheres during preparation. 2. Experimental Section 2.1. Synthesis. 3-Mercaptopropionic acid (MPA, 99%), Cd- (ClO 4 ) 2 , NaBH 4 (96%), and tellurium powder (99.999%, about 200 mesh) were obtained from Sigma, Inc. Ultrapure water with 18.2 MΩ/cm (Millipore Simplicity) was used in all syntheses. The method for the preparation of NaHTe is described elsewhere, 11,12 with a few modifications. Briefly, 100 mg of sodium borohydride was transferred to a small flask, and 2.5 mL of D.I. water was added. After 40 mg of tellurium powder was added to the flask, the reacting flask was rapidly sealed via a rubber plug with a small long syringe pinhead inserted into the flask to discharge pressure from the resulting hydrogen. After 3-8 h, depending on the diameter size of syringe, the black tellurium powder disappeared and a white sodium tetraborate precipitate appeared at the bottom of the flask. The resulting clear pink or colorless aqueous solution was then transferred carefully into 50 mL of degassed water. A series of aqueous colloidal CdTe solutions were prepared using the reaction between Cd 2+ and the NaHTe solution similar to the method described elsewhere. 12 Cd precursor solutions were prepared by mixing a solution of Cd(ClO 4 ) 2 and stabilizer (MPA) solution, and adjusted to different pH values with 2 M NaOH to pH ) 8-8.2. The typical molar ratio of Cd:Te:MPA * Corresponding author. E-mail: weichen@uta.edu. ² Nomadics, Inc. ‡ The University of Texas at Arlington. § Pacific Northwest National Laboratory. | Department of Physics, Oklahoma State University. ⊥ Department of Physiological Sciences, Oklahoma State University. # Lund University. BATCH: jp9a124 USER: ckt69 DIV: @xyv04/data1/CLS_pj/GRP_jp/JOB_i35/DIV_jp063085k DATE: July 28, 2006 10.1021/jp063085k CCC: $33.50 © xxxx American Chemical Society PAGE EST: 8.5 Published on Web 00/00/0000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77