Applications of self-assembled monolayers in Materials Chemistry

Self-assembly provides a simple route to organise suitable organic molecules on noble metal and selected nanocluster surfaces by using monolayers of long chain organic molecules with various functionalities like -SH,-COOH,-NH2, silanes etc. These surfaces can be effectively used to build-up interesting nano level architectures. Flexibility with respect to the terminal functionalities of the organic molecules allows the control of the hydrophobicity or hydrophilicity of metal surface, while the selection of length scale can be used to tune the distant-dependent electron transfer behaviour. Organo-inorganic materials tailored in this fashion are extremely important in nanotechnology to construct nanoelctronic devices, sensor arrays, supercapacitors, catalysts, rechargeable power sources etc. by virtue of their size and shape-dependent electrical, optical or magnetic properties. The interesting applications of monolayers and monolayer-protected clusters in materials chemistry are discussed using recent examples of size and shape control of the properties of several metallic and semiconducting nanoparticles. The potential benefits of using these nanostructured systems for molecular electronic components are illustrated using Au and Ag nanoclusters with suitable bifunctional SAMs

Self-assembled monolayers as a tunable platform for biosensor applications

Considerable attention has been drawn during the last two decades to functionalize noble metal surfaces by forming ordered organic films of few nm to several hundred-nm thickness. Self-assembled monolayer (SAM) provides one simple route to functionalize electrode surfaces by organic molecules (both aliphatic and aromatic) containing free anchor groups such as thiols, disulphides, amines, silanes, or acids. The monolayer produced by self-assembly allows tremendous flexibility with respect to several applications depending upon their terminal functionality (hydrophilic or hydrophobic control) or by varying the chain length (distance control). For example, SAM of long chain alkane thiol produces a highly packed and ordered surface, which can provide a membrane like microenvironment, useful for immobilising biological molecules. The high selectivity of biological molecules integrated with an electrochemical, optical or piezoelectric transduction mode of analyte recognition offers great promise to exploit them as efficient and accurate biosensors. It is demonstrated with suitable examples that monolayer design plays a key role in controlling the performance of these SAM based biosensors, irrespective of the immobilisation strategy and sensing mechanism

Controlled interlinking of Au and Ag nanoclusters using 4-aminothiophenol as molecular interconnects

This work describes the formation of interlinked gold and silver nanoclusters at controlled pH using 4-aminothiophenol (ATP) as a molecular interconnect. UV-visible spectra give on intercrystal plasmon resonance band in the region 550-580 nm. The crystalline heteroassembly formation is also evident from the transmission electron microscopic (TEM) images, whereas X-ray photoelectron spectroscopic (XPS) analysis of the aggregates shows the presence of charged ?N species, indicating electrostatic interaction of ?N with Ag nanoclusters. Furthermore, electrochemical studies of these heteroassembled systems suggest that silver nanoclusters are not fully passivated by the monolayers of ATP and are accessible for redox reactions

Effect of chain length on the tunneling conductance of gold quantum dots at room temperature

Understanding the electronic structure of nanometer-sized metal particles which bridge the gap between the molecules and bulk materials is important due to the fabrication of many nanodevices, like single-electron transistors and molecular switches. Using a simple core-shell model, here, we investigate the variation of electrostatic charging properties of metallic nanoclusters with the chain length of the passivating molecule and cluster core size. The estimation of capacitance (Cc) and charging energy (Ec) as 2Πε0εrRc/(1+2r/Rc) and e2(1+2r/Rc)/4Πε0εrRc, respectively, as a function of the core size as well as the intercluster spacing, reveals that longer chains (8 to 10 methylene units) limit the electron transport beyond usefulness

Role of polyfunctional organic molecules in the synthesis and assembly of metal nanoparticles

The role of polyfunctional organic molecules in the synthesis of differently shaped metallic nanostructures and their assembly is investigated. These molecules could be used as spacer ligands and also for surface passivation of nanoparticles, especially with the objective of controlling their electronic and optical properties depending on their length scales. We investigate the role of several such molecules, such as 4-aminothiophenol, tridecylamine, Bismarck brown R and Y, mordant brown, fat brown, chrysoidin (basic orange), and 3-aminobenzoic acid in the synthesis and assembly of various nanoparticles of gold and silver. For example, the use of 4-ATP helps in the formation of rod shaped micelles in aqueous acetonitrile as confirmed by transmission electron microscopy (TEM) suggesting their role as soft templates. In addition, 4-ATP has also been used for the formation of heteroassembly of spherical nanoparticles of gold and silver at controlled pH. Significantly, triangular and hexagonal gold nanoplates are formed at room temperature by similar polyfunctional dye molecule, Bismarck brown R (BBR), while other analogous dye molecules give only arbitrary shaped gold nanoparticles. Further confirmation of their role in shape determination comes from linear amine molecules such as tridecylamine, which give only spherical nanoparticles both for silver and gold. In essence, our study confirms the role of various such organic molecules in shape controlled synthesis of nanoparticles. We also report optical and electrochemical properties of few of these nanostructures as a function of their shape

Highly resolved quantized double-layer charging of relatively larger dodecanethiol-passivated gold quantum dots

Monolayer-protected quantum dots (Q-dots) show multivalent redox property, popularly known as the quantized double-layer (QDL) charging phenomena. In this report, we demonstrate the QDL behavior of the larger-sized Au Q-dots (ca. 3.72 nm) protected with dodecanethiol using differential pulse voltammetry (DPV) and cyclic voltammetry (CV). The voltammetric results show that the QDL property is evident even for these larger-sized Q-dots as reflected by a large population of well-resolved charging events in a narrow potential range with an almost equidistant voltage (Δ V) spacing. The theoretical calculation of the variation of charging energy with size using the well-known concentric sphere capacitance model facilitates the understanding of electrochemical behavior of these sidelined larger-sized Au Q-dots. The calculated capacitance value is in well agreement with the experimentally obtained value of 1.6 aF

Size dependent redox behavior of monolayer protected silver nanoparticles (2-7 nm) in aqueous medium

Monolayer protected nanoclusters are of current interest due to their ease of synthesis, high stability and possibility to precisely control their aspect ratio by preparation procedures, so that they can be tuned for a wide range of applications. Since these nanostructured metallic particles show fascinating size dependent optical, electronic, catalytic and magnetic properties, it is important to modulate their size, shape and intercluster spacing during their synthesis. These size dependent phenomena suggest that the electrochemistry of nanometer scale metal particles should be different from that of their bulk analogues. In the present study, we report a systematic variation in the redox behaviour of dodecanethiol protected silver nanoparticles with size (2-7 nm). Cyclic voltammograms in 0.1 M aqueous KCl solution show irreversible nature and the redox behaviour is indeed affected by the size as in agreement with the theoretical calculations of the Kubo gap. More specifically, the separation between oxidation and reduction peaks (Δ Ep) increases with an increase in size reaching a maximum (3.5-6 nm) followed by a decline, whereas the E½seems to be almost constant throughout this size regime. As the kinetic parameters are directly related to the ΔEp value, the electron transfer facility should decrease with an increase in size in a similar manner. All the silver nanoclusters were characterized by their surface plasmon peak position, which was found to decrease with increase in size with a concomitant broadening. The particle size calculated from TEM reveals a fairly monodispersed nature whereas selected area electron diffraction (SAED) results confirm the presence of fcc structure for all the Ag clusters

Tuning the aspect ratio of silver nanostructures: the effect of solvent mole fraction and 4-aminothiophenol concentration

In this report, we study the role of solvent on controlling the aspect ratio of silver nanostructures during their growth. More specifically, a single-step preparation of different aspect ratio silver nanostructures (R, 1-100) is demonstrated in aqueous acetonitrile using 4-aminothiophenol (ATP) as a reducing as well as surface passivating agent, where the variation of the mole fraction of acetonitrile has a dramatic effect on the morphology. The combined effect of ATP concentration and solvent mole fraction on aspect ratio is investigated by UV-Visible Spectroscopy (UV-Vis), Transmission Electron Microscopy (TEM), Fourier Transform Infra-red Spectroscopy (FTIR) and X-ray Diffraction analysis (XRD). At lower values of mole fraction (i.e. 0.4), high aspect ratio silver nanorods are formed, whereas a mole fraction close to 1 gives no such nanostructures. In comparison, only spherical nanoparticles are formed when the mole fraction is close to 0. High aspect ratio silver nanorods are also favored by higher ATP concentration