DOI: 10.1021/la100249t 10129 Langmuir 2010, 26(12), 10129–10134 Published on Web 04/30/2010 pubs.acs.org/Langmuir © 2010 American Chemical Society Recovery of CdS Nanocrystal Defects through Conjugation with Proteins Matteo Amelia, † Rebecca Flamini, and Loredana Latterini* Dipartimento di Chimica, Universit  a di Perugia and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN), Via Elce di Sotto 8, 06123 Perugia, Italy. † Present address: Dipartimento di Chimica “G. Ciamician”, Universit  a di Bologna, Via Selmi 2, 40126 Bologna, Italy Received January 18, 2010. Revised Manuscript Received March 19, 2010 The luminescence behavior of CdS nanocrystals in aqueous solution and in the presence of proteins has been deeply investigated. CdS nanocrystals have been prepared in water by thermal decomposition of a single organometallic precursor assisted by thioglycerol, which acts as capping agent. Different experimental conditions have been explored to gain insights into the parameters affecting the nanocrystal growth. The CdS samples were characterized in terms of absorption and emission spectra, luminescence quantum yields, and decay times. These data together with size distribution analysis gave information on the growth mechanism and on the nature of the trap states formed in different experimental conditions. The emission properties of the nanocrystals in the presence of bovine serum albumin (BSA) have been examined to test how the electrostatic bioconjugation can influence the optical properties of the nanocrystals. The spectral changes observed upon addition of BSA indicated a direct interaction of the protein with the nanocrystal surface able to recover (at least partially) the defects formed during the crystal growth, resulting in improved emission properties. Introduction Colloidal semiconductor nanocrystals (or quantum dots, QDs) have attracted a prominent attention in the past three decades since their physical and chemical properties, which can be tuned by size and morphology control during the synthesis, differ from those of their corresponding bulk materials. 1-3 Such properties render these nanomaterials suitable for numerous applications such as solar cell materials, 4,5 light-emitting diodes, 6 optical sensors, 7-9 and bioimaging. 10,11 Their optical properties enabled to consider them as a new class of luminescent probes 3 and QDs have been successfully applied in both single 12 and multicolor techniques 13,14 both in vitro and in vivo conditions in different biological studies. 15 Particularly interesting for biomedical applica- tions appeared the nanocrystals prepared in aqueous media. 16-19 It has been recently reported that the degree of crystallinity and the nanocrystal surface properties and, hence, their luminescence behavior are strongly affected by the reagent nature 20 and by the growth mechanism. 21 In particular, CdS nanocrystals prepared in aqueous solutions using precursors in salt form can enable the formation of trap states which strongly affect the nanocrystal emission properties. 22-24 On the other hand, to achieve biological labeling conjugation of the inorganic nanomaterial with biomolecules is essential. 9,25 Among the methods for linking biomolecules to QDs, noncovalent methodologies present the advantage to be completely reversible, and the binding energy is determined by the chemical environment. 26 *Corresponding author: Tel þþ39-75-5855636; Fax þþ39-75-5855598; e-mail loredana@unipg.it. 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