Layered double hydroxides as carriers for quantum dots@silica nanospheres Georgiana Stoica, * a Iv an Castell o Serrano, a Albert Figuerola, b Irati Ugarte, c Roberto Pacios c and Emilio Palomares * ad Received 20th June 2012, Accepted 1st July 2012 DOI: 10.1039/c2nr31550e Quantum dot–hydrotalcite layered nanoplatforms were successfully prepared following a one-pot synthesis. The process is very fast and a priori delamination of hydrotalcite is not a prerequisite for the intercalation of quantum dots. The novel materials were extensively characterized by X-ray diffraction, thermogravimetry, infrared spectroscopy, transmission electron microscopy, true color fluorescence microscopy, photoluminescence, and nitrogen adsorption. The quantum dot–hydrotalcite nanomaterials display extremely high stability in mimicking physiological media such as saline serum (pH 5.5) and PBS (pH 7.2). Yet, quantum dot release from the solid structure is noted. In order to prevent the leaking of quantum dots we have developed a novel strategy which consists of using tailor made double layered hydrotalcites as protecting shells for quantum dots embedded into silica nanospheres without changing either the materials or the optical properties. Introduction The use of theranostic medicine emerged as a new challenge with respect to the traditional concept about medicine and its appli- cation in human health. Nanoscale materials are becoming more common in the field of medicine, particularly in the field of drug delivery, since they can incorporate and also be functionalized with a wide range of biomolecules. Organic or inorganic plat- forms such as polymers, dendrimers, micelles, vesicles, metals, metal oxides, semiconductor nanocrystals, and nanoparticles have all been already investigated as possible multimodal imaging or simultaneous diagnosis and therapy systems. 1–4 Well-known systems like semiconductor quantum dots (QDs) have attracted great interest in multiplexed bioassays, biotech- nological applications and bioimaging. 5–8 In contrast to tradi- tional fluorophores, QDs possess excellent optical properties, such as continuous absorption profiles, robust signal intensity, narrow emission spectra, and improved brightness with outstanding resistance to photobleaching and degradation. 9–12 Moreover, the development of QDs@nano- or micro-silica spheres as biomolecular probes can provide new insights that overcome several limitations of individual QDs as biological markers, i.e.: better photostability of the embedded QDs in the bead matrix, more available surface for chemical reactions, higher binding capacity of the microspheres, less toxicity, and easier manipulation. 13,14 For example, our own group, recently prepared multicode silica nanospheres of ‘onion’ type with high stability in the biological pH range, i.e. 4–9. 15,16 However, most inorganic nanoparticles require chemical functionalization with silane, thiol, amino and carboxy species in order to obtain desirable properties for cellular delivery, such as good biocompatibility, strong affinity between carrier and payload, cell targeting, stability and long circulation time. 17–19 During the past, it is apparent that layered double hydroxides (LDHs), also known as hydrotalcite-like materials (HT) or anionic clays, form an exception to this rule. LDHs consist of layers of positively charged nanosheets with the brucite-type structure neutralized by anions in the interlayer space, where water is also present. 20,21 Their anion-exchange property 20 allows the direct loading of anionic drugs/biomolecules into the inter- layer galleries. 22–27 LDHs are mostly well-known catalysts and ceramic precursors, and traps for anionic pollutants. 21 The earliest application of hydrotalcites in relation to human health was their use as antiacids and antipeptic reagents, 28–30 whereas in the last decade nanometer-sized LDH materials were increas- ingly explored as drug and gene carriers and controlled release delivery systems. 22,23,25,26,31–37 The capacity to incorporate drugs and other bioactive molecules (peptides, proteins, and nucleic acids) in the interlayer space opened ways for their application in nanomedicine. The first obstacle for the LDH–cargo hybrid materials to be transferred into the cells is the cell membrane, which is hydrophobic and negatively charged. LDH nano- particles, exhibiting a positive surface charge, readily achieves this without necessitating any further surface functionalization, as is the case of silica, 17,18 gold nanoparticles, 19 or carbon a Institute of Chemical Research of Catalonia (ICIQ), Avinguda del Paisos Catalans 16, 43007 Tarragona, Spain. E-mail: epalomares@iciq.es; Fax: +34 977 929 823; Tel: +34 977 920 200 b Department of Inorganic Chemistry, Faculty of Chemistry, Nanoscience and Nanotechnology Institute (IN2UB), University of Barcelona, Mart  ı i Franqu es 1-11, 08028 Barcelona, Spain c IK4-IKERLAN, Goiru Kalea, 20500 Arrasate, Gipuzkoa, Spain d Catalan Institution for Research and Advanced Studies (ICREA), Passeig de Lluis Companys 23, 08010 Barcelona, Spain This journal is ª The Royal Society of Chemistry 2012 Nanoscale Dynamic Article Links C < Nanoscale Cite this: DOI: 10.1039/c2nr31550e www.rsc.org/nanoscale PAPER Downloaded by Imperial College London Library on 25 July 2012 Published on 04 July 2012 on http://pubs.rsc.org | doi:10.1039/C2NR31550E View Online / Journal Homepage