Nanoscale chemistry is the exciting interdisciplinary areas of chemical science and technology where physical and chemical properties of systems addressed depend on their sizes. The size can range from the single molecule (» 1 nm) to many thousands of molecules (> 100 nm), with almost macroscopic properties. Chemical properties at the nanoscale are different from those in macroscopic, say molar scales. Studies of nanoscale systems require high-resolution techniques such as electrochemistry at atomically planar surfaces. Molecules in such environments can be studied by the ultra-sensitive microscopies, scanning tunnelling and atomic force microscopy (STM and AFM, some examples are shown in Figure 1.). We can, literally see the single molecules in action.
Such studies are important because they disclose new chemical behaviour different from the known macroscopic world. They offer further new perspectives for the design and chemical synthesis of novel materials, and for forthcoming electronics and biological screening at ultra-small scales.
Figure 1. Three examples of in situ STM images recorded in liquid environments: (A) “Seeing” individual gold atoms on a Au(111) surface, (B) sub-molecular resolution of L-cysteine molecules self-assembled on a Au(110) surface, and (C) molecular resolution of the Fe-S protein ferredoxin. The bright spots in (A) and (C) represent individual gold atoms and protein molecules, respectively, the latter in full electron transporting action. The unit cell marked by a blue box in B shows an ordered molecular packing lattice, in which three close bright spots correspond to three chemical groups in a single L-cysteine molecule.
DTU Chemistry’s NanoChemistry group offers interesting special course, bachelor, master and Ph.D. projects in different areas of chemistry at the nanoscale listed below. Those interested are welcome to contact us for further information:
Jens Ulstrup ju@kemi.dtu.dk, Qijin Chi qc@kemi.dtu.dk
1. Investigation of Solid / Liquid Electrochemical Interfaces at the Nanometer Scale
The project aims broadly at studying solid / liquid interfaces in an electrochemical environment at the nanometer scale and offers new insight into fundamental understanding of molecular structure and properties in the interfacial region. Two key techniques applied are scanning probe microscopy (SPM), particularly scanning tunnellingmicroscopy (STM) and electrochemistry. The novelty of the project is the combination of high-resolution STM, to study interfacial electrode dynamics and specific adsorption of anions, and molecular assembling under electrochemistry control. Well-defined single crystal electrodes are used throughout in order to reach atomic, submolecular andmolecular levels. In the project, liquid solutions include not only aqueous electrolyte solution but also non-aqueous solution such as ionic liquids.
Figure 2. STM images of single crystal electrode gold surfaces (A) Au(111), (B) Au(100) and (C) Au(110) in ammonium acetate or phosphate solution. Characteristic reconstructions are given under each substrate.
Several specific projects are offered: a) in situ study of electrode surface dynamic under potential control; b) in situ anion adsorption on electrode surfaces; c) construction of metallic nanostructures; d) in situ monolayer formation and dynamic structures; e) study of electrode dynamics in non-aqueous liquid environment.
2. Assembly and structural characterization of biomolecules on singlecrystal metal surfaces
Proteins are composed of 21 L-amino acids. Nature’s specific choice of the L-form is a long-standing puzzle. One hypothesis is life’s origin at chiral solid-liquid interfaces. This project offers studies of amino acid assembly at solid/liquid interfaces, particularly addressing structures on atomically planar metal surfaces with a view of understanding molecular packing, surface interaction and chirality. Technologies are electrochemistry and in situ scanning tunnelling microscopy (STM) to molecular even sub-molecular resolution. Students will learn to prepare single-crystal metallic surfaces and to operate in situ STM at atomic or molecular resolution. The purpose of the project is to achieve a better understanding of how biological molecules interact with and assemble on metallic surfaces, in particular gold and platinum surfaces, at the nanoscale. Figure 1 (middle) shows an example of such a study.
3. Bioelectrochemistry of metalloproteins at the single-molecule level
Electron transfer of redox metalloproteins is of great importance in photosynthesis, respiration and enzyme catalysis. Protein electron transfer and even enzyme kinetics can be studied to an unprecedented level of precision by our group’s technologies. We offer projects that involve the assembly of protein molecules on surfaces, and their characterization by electrochemistry (protein electron exchange with the electrode) and STM/AFM under conditions where the proteins retain their function, Fig.3. Studies of single protein molecules are novel and offer both new insight in fundamental protein properties and a basis for ultra-sensitive biosensing devices. Students will learn to prepare singlecrystal gold surfaces, protein electrochemistry, and to operate STM as well as handling fragile metalloproteins.
Fig.3 Two core proteins studied by our group. Left: A large Cu-enzyme (Cu-NiR) from the biological nitrogen cycle. The spheres show the Cu-atoms. Electrons are let in through the blue centres and transmitted through the protein to the orange centres where the catalytic process occurs. Right: Another Cu-protein, azurin in electron transferring action at a modified Au(111)-electrode surface.
4. Is DNA a molecular conductor?
DNA is known as the bearer of genetic information of living cells. The nucleic acid sequence is the recipe to build proteins needed to perform the ‘daily work’ of the cell. DNA is organized as a double strand of complementary nucleic acids in a helix structure. DNA is also recently recognized (perhaps) as an electric conductor, Figure 4, and electronic conductivity or long-range electron transfer is believed to play a role in the cell’s control mechanisms (DNA repair). Conduction is via the bases. The goals of this project are to investigate these properties using modified DNA at solid surfaces combined with STM and electrochemistry. The results offer perspectives for future bioelectronics and bio-sensing devices. Students will learn to prepare single-crystal surfaces of gold and to operate STM as well as to deal with DNA-based molecule.
Contact person: Jens Ulstrup ju@kemi.dtu.dk.
5. Metallic and inorganic nanoparticles, chemical synthesis, characterization and electrocatalysis application
Metallic nanoparticles (NPs) in the size range 1-20 nm are prime representatives of nanoscience “in action”. Electronic (colour) and chemical properties of, say Au and Pt NPs depend intriguingly on the NP size. Prussian Blue (PB) NPs ([Fe(CN)6]3-/4-) are another example with interesting properties. NPs can be combined into composite structures known as “nanoflowers” etc. We offer projects on the chemical synthesis of metallic and PB NPs and their characterization by the advanced microscopies STM, AFM and transmission electron microscopy (TEM). Catalytic NP properties will be tested in fuel cells and other electrochemical environments. In addition to chemical synthesis, students will learn STM and AFM technology and have access to TEM at DTU’s new microscopy center, CEN. “Hybrids” of metalloproteins and molecular scale inorganic structures such as metallic NPs are new molecular objects with exciting properties that differ from those of the separate metalloproteins and NP components. Binding a protein to a NP also strongly enhances the electron transport between the protein and an electrochemical surface. Projects offered in this new applied nanoscience area include: “functionalization” of metallic NPs and binding of proteins to the functionalized NPs; electrocatalysis of the “hybrids”; and, in situ STM of the hybrids towards the level of resolution of the single hybrid molecular entity. Students will learn these technologies.
Figure 5. left: AFM image of Prussian blue nanoparticles (PBNPs) immobilized on a gold surface (left), and right: comparison of cyclic voltammograms recorded for electrocatalytic activity towards reduction of H2O2 with (red) and without PBNPs (black).
Contact persons: Qijin Chi cq@kemi.dtu.dk and Jens Ulstrup ju@kemi.dtu.dk.
6. Green Synthesis, Characterization and Applications of Metallic Nanostructures
The SAMENS (saccharidebased approach to metallic nanostructure synthesis) method has been recently discovered in the group to produce 8-12 nm gold nanoparticles and 1.6-1.8 nm platinum nanoparticles. This serves as a general platform for “green” synthesis of different nanostructures such as nanoparticles, nanowires, nanosheets and bimetallic nanoflowers shown in Figure 6. The goal of the project is to develop a non-toxic, environmentally friendly method for chemical synthesis of metallic nanostructures with high yield and purity, with shape and size control, and broad application. This project aims for the preparation of not only spherical particles, but also more advanced anisotropic and heteronanostructures such as wires, polygons, “nanoflowers” and other core-shell structures by ”green” chemical synthesis, based on completely harmless reagents. The synthesized structures will be characterized by highresolution techniques, and mapping of electrocatalysis in fuel cell and bioelectrochemical environments. Several special courses, bachelor project and master project have been carried out successfully on this topic. One Ph.D project (2011-2014) is running along this line.
Figure 6 Nanostructures synthesized by the green method. (A) AuNPs, (B) PtNPs, (C) FeOx nanoparticles, (D) Au nanowire, (E) Pt nanowires, (F) Au nanosheet and (G) Au/Pt nanoflower. D and E are AFM images, the rest are TEM pictures.
Contact persons: Christian Engelbrekt (cheng@kemi.dtu.dk), Jingdong Zhang (jz@kemi.dtu.dk).
7. Theory of molecular and nanoscale electron transport
Electrical currents and electron transfer between molecules is a key notion in nano- and molecular-scale science in most of the projects above. Rational approaches to this nanoworld of new phenomena require theoretical frames to understand the phenomena and offer a guide to optimal molecular function. Theoretical methods in the projects address for example molecular monolayers, single redox biomolecules and redox metalloenzymes, as well as inorganic NP/metalloprotein hybrids. We offer interesting projects also in these areas of theoretical chemical nanoscale science. Fig.7 shows three
examples of single-molecule behaviour for which new theoretical frames are needed.
Figure 7. Top left: An electron transfer protein in action at a Au(111) electrode surface. The corresponding electrochemical signal is also shown. Top right: A large redox metalloenzyme (laccase) with four Cu-atoms. The spots in bot pictures are single protein molecules in electron transfer or enzyme action. Bottom: A 1.5 nm AuNP (Au145) can be successively charged by single-electron transfer. Is the NP a molecule or a small chunk of metal
Contact person: Jens Ulstrup ju@kemi.dtu.dk.
8. Chemical preparation and applications of graphene nanomaterials
As the thinnest two-dimensional material, graphene is emerging as a new star of carbon nanomaterials. Its unique electronic, optical and mechanical properties have triggered studies world wide with promising applications in areas of electronic devices, chemical and biological analysis, and energy conversion and storage. The projects collectively aim to develop new chemical methods for preparation of graphene nanomaterials and to use them for chemical and biological sensing in chemical environments.
Figure 8. Comparison of the structures for interesting carbon nanomaterials including graphene, carbon nanotube, graphite and C60 (Left) and Atomic-scale resolution of graphene nanosheets imaged by STM and (right).
Contact person: Qijin Chi, E-mail : cq@kemi.dtu.dk; phone : 45252032
9. Graphene–nanoparticle composite materials for energy conversion applications
Energy and environment are of global concern, which has stimulated worldwide research on efficient and clean energy technologies. Among the arising technologies are fuel cells such as the proton exchange membrane fuel cells (PEMFC). However, performance, service life and costs are major bottlenecks for this technology. The major voltage loss (about 80%) of a PEMFC is due to slow kinetics of dioxygen reduction at the cathode[1], while noble metal catalysts account for a significant part of the costs. Graphene, a two-dimensional single-atomically planar sheet of sp2 bonded carbon atoms
densely packed into a honeycomb lattice structure, has attracted intense scientific attention since its discovery in 2004 [2]. Graphene possesses unique mechanical properties, such as high elasticity, very high mechanical strength, and high surface area, as well as interesting electronic properties such as high conductivity in particular. Recent studies have shown that graphene can be combined with metallic nanoparticles to form novel materials with exciting structural, mechanical, optical, electronic and catalytic properties. This new class of nanocomposite materials is expected to have a significant impact as catalysts in fuel cell processes, where the graphene part will serve as highly conductive and chemically stable loading material for small size noble metal nanoparticles. The project will aim at synthesizing and characterizing graphene-nanoparticle composite materials in the context of fuel cell applications.
Contact persons: Jens Ulstrup ju@kemi.dtu.dk, Jingdong Zhang jz@kemi.dtu.dk
References
[1] R. Hiesgen, D. Eberhardt, E. Aleksandrova, K. A. Friedrich, Fuel Cells, 2006, 6, 425
[2] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666