Sudhanshu Srivastava

Composite Layer-by-Layer (LBL) Assembly with Inorganic Nanoparticles and Nanowires

New assembly techniques are required for creating advanced materials with enough structural flexibility to be tuned for specific applications, and to be practical, the techniques must be implemented at relatively low cost. Layer-by-layer (LBL) assembly is a simple, versatile, and significantly inexpensive approach by which nanocomponents of different groups can be combined to coat both macroscopically flat and non-planar (e.g., colloidal core-shell particles) surfaces. Compared with other available assembly methods, LBL assembly is simpler and more universal and allows more precise thickness control at the nanoscale. LBL can be used to combine a wide variety of species--including nanoparticles (NPs), nanosheets, and nanowires (NWs)--with polymers, thus merging the properties of each type of material. This versatility has led to recent exceptional growth in the use of LBL-generated nanocomposites. This Account will focus on the materials and biological applications of introducing inorganic nanocrystals into polymer thin films. Combining inorganic NPs and NWs with organic polymers allows researchers to manipulate the unique properties in the nanomaterial. We describe the LBL assembly technique for introducing metallic NPs into polymers in order to generate a material with combined optomechanical properties. Similarly, LBL assembly of highly luminescent semiconductor NPs like HgTe or CdTe with poly(diallyldimethylammonium chloride) (PDDA) was used to create uniform optical-quality coatings made on optical fibers and tube interiors. In addition, LBL assembly with inorganic nanosheets or clay molecules is reported for fabricating films with strong mechanical and ion transport properties, and the technique can also be employed to prepare Au/TiO(2) core/sheath NWs. The LBL approach not only will be useful for assembly of inorganic nanocrystals with various polymers but can be further applied to introduce specific functions. We discuss how the expanded use of NWs and carbon nanotubes (CNTs) in nanocomposite materials holds promise in the design of conductive films and new nanoscale devices (e.g., thin-film transistors). New photonic materials, sensors, and amplifiers can be constructed using multilayer films of NPs and can enable fabrication of hybrid devices. On the biological side, inorganic nanoshells were used as assembly tools with the goal of detecting neurotransmitters (specifically, dopamine) directly inside brain cells. In addition, the stability of different cell lines was tested for fabricating biocompatible films using LBL. NP LBL assembly was also used for homogeneous and competitive fluorescence quenching immunoassay studies for biotin and anti-biotin immunoglobulin molecules. Finally, introduction of biomolecules with inorganic NPs for creating biocompatible surfaces could also lead to new directions in the field of biomedical applications.

Light-Controlled Self-Assembly of Semiconductor Nanoparticles into Twisted Ribbons

Nanoparticles, Lightly Twisted The helical structures that are widespread in natural macromolecules result from well-coordinated bonding interactions and affect their physical properties in striking ways. To obtain helical nanoparticles, Srivastava et al. (p. 1355, published online 11 February) slowly oxidized cadmium-tellurium under visible light and assembled ribbons of nanostructure. The ribbons were persuaded to twist into helices because they were doped with cadmium sulfide nanoparticles, which underwent surface oxidation and caused localized stresses that could only be relieved by a conformational change. The pitch of the twisted ribbons that were produced could be controlled by the intensity of illumination applied. This behavior offers promise for application in the development of materials with interesting optical properties. The photooxidation of cadmium sulfide nanoparticles within cadmium telluride nanoparticle ribbons causes surface stresses that lead to twisting. The collective properties of nanoparticles manifest in their ability to self-organize into complex microscale structures. Slow oxidation of tellurium ions in cadmium telluride (CdTe) nanoparticles results in the assembly of 1- to 4-micrometer-long flat ribbons made of several layers of individual cadmium sulfide (CdS)/CdTe nanocrystals. Twisting of the ribbons with an equal distribution of left and right helices was induced by illumination with visible light. The pitch lengths (250 to 1500 nanometers) varied with illumination dose, and the twisting was associated with the relief of mechanical shear stress in assembled ribbons caused by photooxidation of CdS. Unusual shapes of multiparticle assemblies, such as ellipsoidal clouds, dog-bone agglomerates, and ribbon bunches, were observed as intermediate stages. Computer simulations revealed that the balance between attraction and electrostatic repulsion determines the resulting geometry and dimensionality of the nanoparticle assemblies.

Nanoparticle assembly for 1D and 2D ordered structures

The skill to use new synthetic routes to assemble nanoparticles (NPs) into advanced architectures for specific functions is a big challenge for the researchers. Controlled morphologies designed for generating 1-dimensional (1D) and 2-dimensional (2D) structures inspired by nature are one of the hot topics in nanoscience. The unique properties of individual nanocrystals upon assembly also provide the opportunity to modulate or enhance the overall behavior of the nanomaterials. New synthetic approaches will be discussed to explore the ability of the NP (metallic, semiconductor and magnetic) assembly in various highly ordered superstructures. The mechanism of NP assembly (non-covalent or covalent) with itself or by targeting polymers and biomolecules as linkers will also be illustrated. The topics will include new methods for construction of 1D nanowires and 2D nanosheets by spontaneous dipole–dipole interactions, air–water interface assembly and specific interactions using polymer and bio-templates. Use of polymers as templates for generating arrays of NPs or biomolecules (for investigating the intrinsic behavior of DNA or proteins in the assembly) opens the possibility of designing novel devices and hybrid materials. Once the desired superstructures have been fabricated and characterized, the corresponding materials can be further investigated for optical, magnetic, electronic and sensor applications.

Reversible loading and unloading of nanoparticles in "exponentially" growing polyelectrolyte LBL films.

The exponentially growing layer-by-layer (LBL) films made from poly(diallyldimethylammonium chloride) (PDDA) and poly(acrylic acid) (PAA) were used to load and unload the CdTe nanoparticles (NPs). The reversible loading of NPs were investigated through UV-vis studies and further confirmed by confocal microscopy. In addition the LBL films were also compared for the release kinetics for pH 9 and 7 and films capped with (PDDA-PSS)10 layers. The amount of released particles at pH 9 was found to be at least 2 orders of magnitude higher than those at pH 7 and with (PDDA-PSS)10 capped layers after 25 h. This variation in film response for CdTe-particle release presents a route for studies in which highly swollen exponentially growing LBL films can be loaded with functionalized NPs for biological applications and explored as carriers to hold the NPs inside the films for self-assembly.

Diffusional Self-Organization in Exponential Layer-By-Layer Films with Micro- and Nanoscale Periodicity

The layer-by-layer (LBL) assembly technique is currently one of the most widely utilized methods for the preparation of nanostructured, multilayered thin films. The structure of LBL films is typically controlled by varying the deposition sequence of adsorbed layers, leading to stratified assemblies. For specific, non-spherical inorganic LBL components, such as sheets, or axial nanocolloids, such as nanotubes, nanowires, nanowiskers, or nanorods, the structure of the films can also be controlled by their orientation. As such, clay nanosheets spontaneously adsorb almost exclusively in the orientation parallel to the substrate, whilst assembly of axial nanocolloids under conditions of shear or dewetting results in partial alignment of the fibrous components. Morphological or structural control of the multilayers can also be imparted by the choice of the assembly method (e.g. spin coating versus dip coating), the assembly conditions, or post-assembly processing of the assembly. The shape and surface morphology of the assemblies can also be tailored by the structure or shape of the substrate, as has been shown in the preparation of hollow capsules or sculptured/perforated membranes. Both polymers and nanoparticles exhibit strong tendencies toward self-organization. This effect has not been utilized in the LBL assemblies, except for the recent observation by Yoo et al. of the organization of rod-shaped viruses on the surface of a film consisting of a few bilayers. Overall, the need for more sophisticated degrees of structural organization is quite extensive and commensurate with the increasingly complex applications for which they are being prepared. Importantly, this control must be possible on a nanometer and a micrometer scale. In principle, the LBL approach does allow such a broad-scale control, but microscale films require deposition of a great number of layers in traditional LBL. It would be exceptionally advantageous to design a method that can lead to well-organized materials combining fast deposition and hierarchical nano-, micro-, and macroscopic levels of organization. To achieve this aim, a degree of smartness and the presence of elements of selforganization in the film will most likely be required. Layered systems with alternating microand nanostrata of a stiff and an elastic nature might be particularly interesting because of mechanical properties associated with the distribution of stress in hierarchical structures and predicted theoretically unique mechanical properties. Exponentially grown LBL (e-LBL) films are multilayers in which polymer chains retain their mobility and diffuse through the deposited strata. The degree of mobility makes possible to observe self-organization phenomena in such structures. Herein, we show that a system with alternating nanometerand micrometer-scale layers of predominantly inorganic (stiff) and polymeric (elastic) layers forms upon LBL deposition of poly(diallyldimethylammonium chloride) (PDDA), poly(acrylic acid) (PAA), and sodium montmorillonite clay nanosheet (MTM) multilayers. Despite the expectations of fairly homogeneous coatings in the framework of both traditional and exponential LBL deposition, the deposition sequence (PDDA/PAA/PDDA/MTM)n (n is the number of deposition cycles), results in well-defined indexing of the films after the first few cycles, with a periodicity of (1.7 0.4) mm for 10 min deposition, and superimposed organization of MTM sheets at the interfaces with 0–10 nm spacing (Figure 1). The indexing can be further controlled by varying the deposition times for polyelectrolytes (Supporting Information, Figure S1). A typical assembly included alternate immersion of a glass slide into solutions of the polycation PDDA and an [*] Prof. N. A. Kotov Departments of Chemical Engineering, Materials Science and Engineering, and Biomedical Engineering University of Michigan, Ann Arbor, MI 48109 (USA) Fax: (+1)734-764-7453 E-mail: kotov@umich.edu

Terminal supraparticle assemblies from similarly charged protein molecules and nanoparticles

Self-assembly of proteins and inorganic nanoparticles into terminal assemblies makes possible a large family of uniformly sized hybrid colloids. These particles can be compared in terms of utility, versatility and multifunctionality to other known types of terminal assemblies. They are simple to make and offer theoretical tools for designing their structure and function. To demonstrate such assemblies, we combine cadmium telluride nanoparticles with cytochrome C protein and observe spontaneous formation of spherical supraparticles with a narrow size distribution. Such self-limiting behaviour originates from the competition between electrostatic repulsion and non-covalent attractive interactions. Experimental variation of supraparticle diameters for several assembly conditions matches predictions obtained in simulations. Similar to micelles, supraparticles can incorporate other biological components as exemplified by incorporation of nitrate reductase. Tight packing of nanoscale components enables effective charge and exciton transport in supraparticles and bionic combination of properties as demonstrated by enzymatic nitrate reduction initiated by light absorption in the nanoparticle.

Helical Assemblies of Gold Nanoparticles

Chiral metallic nanohelices with periodicity in the submicrometer regime are made from twisted CdTe nanoribbons. These helices are highly effective in polarization rotation at wavelengths centered at their pitch length. Interestingly, the chiro-optical performance of helical nanoparticle assemblies is better than that of solid gold nanowires with similar geometries, which sets an important target for the design of metamaterials.

Spontaneous self-organization enables dielectrophoresis of small nanoparticles and formation of photoconductive microbridges

Detailed understanding of the mechanism of dielectrophoresis (DEP) and the drastic improvement of its efficiency for small size-quantized nanoparticles (NPs) open the door for the convergence of microscale and nanoscale technologies. It is hindered, however, by the severe reduction of DEP force in particles with volumes below a few hundred cubic nanometers. We report here DEP assembly of size-quantized CdTe nanoparticles (NPs) with a diameter of 4.2 nm under AC voltage of 4-10 V. Calculations of the nominal DEP force for these NPs indicate that it is several orders of magnitude smaller than the force of the Brownian motion destroying the assemblies even for the maximum applied AC voltage. Despite this, very efficient formation of NP bridges between electrodes separated by a gap of 2 μm was observed even for AC voltages of 6 V and highly diluted NP dispersions. The resolution of this conundrum was found in the intrinsic ability of CdTe NPs to self-assemble. The species being assembled by DEP are substantially bigger than the individual NPs. DEP assembly should be treated as a process taking place for NP chains with a length of ~140 nm. The self-assembled chains increase the nominal volume where the polarization of the particles takes place, while retaining the size-quantized nature of the material. The produced NP bridges were found to be photoactive, producing photocurrent upon illumination. DEP bridges of quantum confined NPs can be used in fast parallel manufacturing of novel MEMS components, sensors, and optical and optoelectronic devices. Purposeful engineering of self-assembling properties of NPs makes possible further facilitation of the DEP and increase of complexity of the produced nano- and microscale structures.

Soft X‐Ray Microscopic Investigation on Self Assembling Nanocrystals

Soft x‐ray microscopy is well suited to investigation of nanoparticles in liquid media. Using a table‐top microscope based on a gas‐discharge source emitting at 2.88 nm, dried CdTe nanowires and dried PbS hyperbranched nanocrystals are investigated. These structures are the result of the assembly of nanoparticles in a liquid environment. Soft x‐ray microscopy is aiming at a better understanding of underlying parameters that affect the self assembly to the desired final structures. It is shown that the presented setup is able to image these particles with a resolution of about 50 nm with exposure times in the range of tens of seconds. The discharge source has a photon flux of more than 109u2009photons/(sru2009su2009μm2) at a photon energy of 430 eV with a bandwidth of λ/Δλu2009=u2009840. The repetition rate of the source is up to 1000 Hz. With the current setup a photon flux of 5×106u2009photons/(μm2u2009s) at the sample is achieved.

Layer-by-layer (LBL) assembly with semiconductor nanoparticles and nanowires

The length scales in nanometer range for both inorganic and organic materials have unique physical responses. The opto-electronic properties of metals [1] and semiconductors [2] strongly depend on their size, material used and crystalline shape in the nanometer size regime. New synthetic approaches have been applied to fabricate series of monodisperse nanometer size crystals, known as nanocrystals or nanoparticles (NPs). In particular, semiconductor NPs [3] have attracted tremendous attention due to their unique physical properties, which originate from the surface atoms of nanoscale objects and from the size quantization effect [4]. Group II–VI semiconductor NPs are currently of great technological interest as emitting materials for thin film electroluminescence devices [5], [6] and as optical amplifier media for telecommunication networks [7], [8], because of their strong bandgap luminescence. The incorporation of luminescent semiconductor NPs into photonic crystals [9] has also obtained substantial attention recently as a novel light source with controllable spontaneous emission. Thiol-capped semiconductor NPs with size-dependent luminescence in the visible spectral region have been synthesized in aqueous colloidal solutions [10] and used for fabrication of light-emitting diodes (LEDs) [11], and for impregnation of colloidal photonic crystals [12].