Wolfgang J. Parak

Biological applications of gold nanoparticles

This critical review gives a short overview of the widespread use of gold nanoparticles in biology. We have identified four classes of applications in which gold nanoparticles have been used so far: labelling, delivering, heating, and sensing. For each of these applications the underlying mechanisms and concepts, the specific features of the gold nanoparticles needed for this application, as well as several examples are described (142 references).

Surface modification, functionalization and bioconjugation of colloidal inorganic nanoparticles

Inorganic colloidal nanoparticles are very small, nanoscale objects with inorganic cores that are dispersed in a solvent. Depending on the material they consist of, nanoparticles can possess a number of different properties such as high electron density and strong optical absorption (e.g. metal particles, in particular Au), photoluminescence in the form of fluorescence (semiconductor quantum dots, e.g. CdSe or CdTe) or phosphorescence (doped oxide materials, e.g. Y2O3), or magnetic moment (e.g. iron oxide or cobalt nanoparticles). Prerequisite for every possible application is the proper surface functionalization of such nanoparticles, which determines their interaction with the environment. These interactions ultimately affect the colloidal stability of the particles, and may yield to a controlled assembly or to the delivery of nanoparticles to a target, e.g. by appropriate functional molecules on the particle surface. This work aims to review different strategies of surface modification and functionalization of inorganic colloidal nanoparticles with a special focus on the material systems gold and semiconductor nanoparticles, such as CdSe/ZnS. However, the discussed strategies are often of general nature and apply in the same way to nanoparticles of other materials.

Antibacterial properties of nanoparticles

Antibacterial agents are very important in the textile industry, water disinfection, medicine, and food packaging. Organic compounds used for disinfection have some disadvantages, including toxicity to the human body, therefore, the interest in inorganic disinfectants such as metal oxide nanoparticles (NPs) is increasing. This review focuses on the properties and applications of inorganic nanostructured materials and their surface modifications, with good antimicrobial activity. Such improved antibacterial agents locally destroy bacteria, without being toxic to the surrounding tissue. We also provide an overview of opportunities and risks of using NPs as antibacterial agents. In particular, we discuss the role of different NP materials.

Biological applications of magnetic nanoparticles.

In this review an overview about biological applications of magnetic colloidal nanoparticles will be given, which comprises their synthesis, characterization, and in vitro and in vivo applications. The potential future role of magnetic nanoparticles compared to other functional nanoparticles will be discussed by highlighting the possibility of integration with other nanostructures and with existing biotechnology as well as by pointing out the specific properties of magnetic colloids. Current limitations in the fabrication process and issues related with the outcome of the particles in the body will be also pointed out in order to address the remaining challenges for an extended application of magnetic nanoparticles in medicine.

Biological applications of colloidal nanocrystals

Due to their interesting properties, research on colloidal nanocrystals has moved in the last few years from fundamental research to first applications in materials science and life sciences. In this review some recent biological applications of colloidal nanocrystals are discussed, without going into biological or chemical details. First, the properties of colloidal nanocrystals and how they can be synthesized are described. Second, the conjugation of nanocrystals with biological molecules is discussed. And third, three different biological applications are introduced: (i) the arrangement of nanocrystal–oligonucleotide conjugates using molecular scaffolds such as single-stranded DNA, (ii) the use of nanocrystal–protein conjugates as fluorescent probes for cellular imaging, and (iii) a motility assay based on the uptake of nanocrystals by living cells.

Synthesis, Characterization, and Bioconjugation of Fluorescent Gold Nanoclusters toward Biological Labeling Applications

Synthesis of ultrasmall water-soluble fluorescent gold nanoclusters is reported. The clusters have a decent quantum yield, high colloidal stability, and can be readily conjugated with biological molecules. Specific staining of cells and nonspecific uptake by living cells is demonstrated.

Prospects of Nanoscience with Nanocrystals

Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Todays strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenomena are constantly being discovered in the photophysics of NCs and in the electronic properties of NC solids. In this Nano Focus, we review the state of the art in research on colloidal NCs focusing on the most recent works published in the last 2 years.

Labelling of cells with quantum dots

Colloidal quantum dots are semiconductor nanocrystals well dispersed in a solvent. The optical properties of quantum dots, in particular the wavelength of their fluorescence, depend strongly on their size. Because of their reduced tendency to photobleach, colloidal quantum dots are interesting fluorescence probes for all types of labelling studies. In this review we will give an overview on how quantum dots have been used so far in cell biology. In particular we will discuss the biologically relevant properties of quantum dots and focus on four topics: labelling of cellular structures and receptors with quantum dots, incorporation of quantum dots by living cells, tracking the path and the fate of individual cells using quantum dot labels, and quantum dots as contrast agents.

Biodistribution of PEG-modified gold nanoparticles following intratracheal instillation and intravenous injection.

Besides toxicity tests, biokinetic studies are a fundamental part of investigations to evaluate a safe and sustainable use of nanoparticles. Today, gold nanoparticles (Au NPs) are known to be a versatile tool in different areas such as science, engineering or medicine. In this study, we investigated the biokinetics after intravenous and intratracheal applications of poly(ethylene glycol) (PEG) modified Au NPs compared to plain Au NPs. Radioactive-labeled Au NPs of 5 nm inorganic core diameter were applied to rats and the NP content in tissues, organs and excretion were quantified after 1-hour and 24-hours. After intravenous injection, a prolonged blood circulation time was determined for Au NPs with 10 kDa PEG chains. Non-PEGylated Au NPs and 750 Da PEG Au NPs accumulated mostly in liver and spleen. After intratracheal application the majority of all three types of applied NPs stayed in the lungs: the total translocation towards the circulation did not differ considerably after PEGylation of the Au NPs. However, a prolonged retention time in the circulation was detected for the small fraction of translocated 10 kDa PEG Au NPs, too.

Design of an amphiphilic polymer for nanoparticle coating and functionalization.

Inorganic colloidal nanoparticles, such as quantum dots or Au nanoparticles, have been extensively investigated for two decades in physics as well as in chemistry. Applications in a variety of fields such as optics, electronics, and biology are envisaged and important proof-of-concept studies have been reported. In particular, with regard to biologically motivated applications, colloidal stability is a key requirement. Apart from nanoparticles stabilized with small ligand molecules, lipids, [6–8] and surface silanization, amphiphilic polymers have been also used by several groups to disperse originally hydrophobic nanoparticles in aqueous solution. This class of amphiphilic particle coatings not only enables the phase transfer of the nanoparticles from organic solvents to aqueous solution, but also serves as a versatile platform for chemical modification and bioconjugation of the particles because biological molecules can be covalently linked to the polymer surface. Because the stability of the amphiphilic coating around the nanoparticle solely depends on the hydrophobic interaction, this procedure is very general and does, for example, not depend on the material of the inorganic nanoparticle core, as it is the case for ligand exchange protocols. Because of the numerous contact points mediated by hydrophobic interaction, the attachment of the polymer to the particle surface is highly stable and can be improved further by crosslinking of the polymer shell. Nowadays quantum dots coated with amphiphilic polymers and with various biological molecules attached to their surface are commercially available (e.g., Invitrogen). The amphiphilic polymers that have been used so far for coating hydrophobic inorganic nanoparticles consist of hydrophobic side chains for the linkage to the nanoparticle surface and a hydrophilic backbone that provides water solubility through charged groups (in general -COO ) and also acts as an anchor for the attachment of biological molecules with bioconjugate chemistry. In this report, we introduce an amphiphilic polymer which involves a third kind of building block: functional organic molecules. The functional organic molecules are linked to the hydrophobic side chains in a similar way as the hydrophilic backbone and provide additional functionality in the particle surface (Figure 1). The amphiphilic polymer described here is based on a poly(maleic anhydride) backbone. Reaction of a fraction of the anhydride rings with alkylamines leads to the formation of the hydrophobic side chains that are needed for intercalation with the hydrophobic surfactant layer on the nanoparticle surface. Another fraction of the anhydride rings is used to link functional organic molecules to the backbone. Like the alkylamines, organic molecules bearing amino-groups can be directly linked to the anhydride rings by reaction of the anhydride with the amino group. In this way alkylamines and organic molecules with amino terminations can be linked to the polymer backbone in a one-pot reaction. The resulting amphiphilic polymer is then wrapped around hydrophobic capped nanoparticles and the organic solvent is replaced by aqueous solution according to our previously published procedure. By linking some of the remaining anhydride rings with diamine linkers, the polymer molecules around each nanoparticle are interconnected and, thus, the shell is crosslinked. Upon phase transfer to aqueous solution, the remaining anhydride rings open to yield negatively charged carboxyl groups, which provide electrostatic repulsion resulting in a stable dispersion of the nanoparticles. Apart from negatively charged carboxyl groups, the polymer surface of the nanoparticles also contains embedded functional organic molecules. The strategy reported here has several advantageous features: 1) The maleic anhydride moieties react spontaneously with high yield with both amino-modified hydrophobic side-chains (such as alkylamines) and functional organic molecules with amino terminal groups. 2) No additional reagents are needed for the coupling. In comparison, [*] R. A. Sperling, M. Zanella, Prof. W. J. Parak Fachbereich Physik, Philipps Universit#t Marburg Renthof 7, 35037 Marburg (Germany) E-mail: Wolfgang.Parak@physik.uni-marburg.de C.-A. J. Lin, R. A. Sperling, P.-Y. Li, M. Zanella, Prof. W. J. Parak Center for NanoScience Ludwig-Maximilians-Universit#t M8nchen Munich (Germany) C.-A. J. Lin, T.-Y. Yang, W. H. Chang Department of Biomedical Engineering Chung Yuan Christian University Taiwan (ROC) C.-A. J. Lin, J. K. Li, W. H. Chang R&D Center for Membrane Technology Center for Nano Bioengineering Chung Yuan Christian University Taiwan (ROC) [] These authors contributed equally to this work. [] Present address: Institute of Biotechnology, National Cheng Kung University, Taiwan (ROC)