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Microchannel process technology (MPT) offers several advantages to the field of nanomanufacturing: 1) improved process control over very short time intervals owing to shorter diffusional distances; and 2) reduced reactor size due to high surface area to volume ratios and enhanced heat and mass transfer. The objective of this thesis was to consider how nanomaterials, produced in part using MPT, could be used to solve problems associated with the fabrication of MPT devices. Specifically, many MPT devices are produced using transient liquid-phase brazing involving an electroplated interlayer consisting of a brazing alloy designed for melting temperature suppression. Unfortunately, these alloys can form brittle secondary phases which significantly reduce bond strength. In contrast, prior efforts have shown that it is possible to leverage the size-dependent properties of nanomaterials to suppress brazing temperatures. In this prior work, thin films of off-the-shelf elemental nanoparticles were used as interlayers yielding joints with improved mechanical properties. In the present investigation, efforts have been made to characterize the synthesis and deposition of various elemental nanoparticle suspensions for use in the transient liquid-phase brazing of aluminum and stainless steel. Advances were used to demonstrate the nanoparticle-assisted diffusion brazing of a microchannel array. In the first section, a silver nanoparticle (AgNP) interlayer was produced for the diffusion brazing of heat exchanger aluminum. Efforts are made to examine the effect of braze filler particle size (~5 nm and ~50 nm) and processing parameters (heating rate: 5ºC/min and 25ºC/min; brazing temperature: 550ºC and 570ºC) on thin coupons of diffusion-brazed 3003 Al. A tensile strength of 69.7 MPa was achieved for a sample brazed at 570°C for 30 min under 1 MPa with an interlayer thickness of approximately 7 μm. Further suppression of the brazing temperature to 500ºC was achieved by sputtering a 1 µm thick layer of Cu before depositing a 5 nm thick film of AgNPs resulting in a lap shear strength of 45.3±0.2 MPa. In the middle section of this thesis, several techniques are investigated for the synthesis of sub 10 nm diameter nickel nanoparticles (NiNPs) to be used in the diffusion brazing of 316L stainless steel. The average NiNP size was varied from 9.2 nm to 3.9 nm based on the synthesis technique, solvent and reducing agent used. Conventional wet-chemical synthesis using NiCl₂.6H₂O in ethylene glycol (solvent) and N₂H₄.H₂O (reducing agent) resulted in the formation of 5.4 ± 0.9 nm NiNPs. Continuous flow synthesis using a microchannel T-mixer (barrel diameter of 521µm) and a 10 second residence time of reactants in a bath temperature of 130ºC resulted in a particle size of with 5.3 ± 1 nm. To make the synthesis safer and less energy intense, microwave heating was used along with less toxic Ni(CH₃CO₂)₂·4H₂O (nickel salt), propylene glycol (solvent) and NaPH₂O₂ (reducing agent) yielding 3.9 ± 0.8 nm diameter NiNPs. For the final section, nickel nanoparticles were synthesized using NiCl₂.6H₂O (nickel salt), de-ionized water (solvent), NaBH₄ (co-reducing agent), N₂H₄.H₂O (reducing agent) and polyvinylpyrolidone (capping agent) yielding 4.2 ± 0.6 nm NiNP. Several deposition techniques were investigated for controlling film thickness and uniformity in the diffusion brazing of 316L stainless steel (SS). Using in-house prepared NiNP and automated dispensing, a hermetic joint up to 70 psi (tested pressure) was obtained in 316L SS substrates under brazing conditions of 800ºC, 2 MPa and 30 min. Throughout the course of this thesis, techniques used for characterizing nanoparticles, films and joints included FT-IR, XRD, SEM, TEM, HRTEM, EDS, EPMA, DSC, mass spectrometry, and lap-shear testing. / Graduation date: 2012

Nanoparticles are used in various applications like medical diagnostics, drug delivery, energy technology, electronics, catalysis etc. Although particles of such small dimensions can be synthesized through various methods, the liquid phase synthesis methods stands out for their simplicity. Typically, these methods involve reaction of precursors to form solute. At high concentration of solute, nucleation commences and nuclei are formed. These nuclei grow in size by assimilating solute from the bulk. Stabilizers or capping agents compete with solute for adsorption on the surface of a growing particle. Two partially protected particles can form bigger particle by coagulation. Uncontrolled turbulent flow field in laboratory scale reactors combined with all the above quite fast and poorly understood steps often lead to poorly controlled synthesis of particles. In many a systems, it also leads to very poor reproducibility. Any attempt to synthesis nanoparticles at engineering scale, with good control on mean size and polydispersity, requires quantitative understanding of the synthesis process. It can then be combined with description of other transport processes in reactors to optimize synthesis protocols. Two main factors hinder progress in this direction: complex and often poorly understood chemistry, and inefficient tools to simulate particle synthesis. In the first part of the thesis, a quantitative model is developed for tannic acid method of synthesis of gold nanoparticles. It showcases the approach used to model a system with poorly understood chemistry and which defies an understanding through the widely used homogeneous nucleation based mechanism for particle synthesis. An organizer based mechanism in which tannic acid brings together nucleating species to facilitate nucleation is invoked. Simple reaction network based models however fail to explain the experimental findings. The underlying chemistry is explored to develop a comprehensive reaction network. This network is used as a guide to seek pathways which can mimic burst of nucleation, a characteristic of homogeneous nucleation based mechanism, through self-limiting nucleation, and various other features present in the experimental data. After successful prediction of all the features of the experimental data through this network, a minimal organizer based mechanism which leads to self-limiting nucleation is developed. The minimal organizer model offers itself as a competing and alternative mechanism to explain nanoparticle synthesis. A few new predictions made by the new model are verified by others in our group. Monte-Carlo (MC) simulations are used as a powerful tool to simulate stochastic processes. Their application to nanoparticle synthesis is limited by three problems: (i) zero initial rate of stochastic processes which leads to infinite time step at the beginning of the simulation, (ii) sensitively time dependent rate of stochastic processes, and (iii) computation intensive simulations. We propose a new approach to carry out MC simulations. It makes use of simulation results obtained with systems of extremely small sizes. These system size dependent predictions, obtained at substantially reduced computational cost are used to construct results for system of infinite size. The approach is based on a new power law scaling that we have found in this work. An efficient implementation of MC simulation for time dependent rate processes is also developed. In this method, an additional variable is introduced for inter-event evolution. It increases the number of differential equation by one, but significantly reduces the computational effort required to estimate the interval of quiescence for time dependent rate processes. All the above ideas are combined in the new approach to simulate complete size distribution for simultaneous nucleation and growth of nanoparticles for a system of infinite size from erroneous simulations carried out with three extremely small size systems. A new framework for solving multidimensional population balance equations (PBEs) which routinely arise in modeling of nanoparticle synthesis is also developed. The new framework advances the concept of minimal internal consistency of discretization. It suggests that an n dimensional PBE is a statement of evolution of population of particles while accounting for how n internal attributes of particles change in particulate events. Thus, a minimum of n + 1 attributes of particles, instead of 2n attributes used hitherto, need to be represented perfectly in discrete representation. This is termed as the concept of minimum internal consistency of discretization in this work. The elements used for discretization should therefore be triangles for 2-d, tetrahedrons for 3-d, and an object with n + 1 vertices in n-d space for the solution of a n-d PBE. The results presented for the solutions for 2-d and 3-d PBEs show the superiority of this framework over the earlier framework. The present work also shows that directionality of elements plays a critical role in the solution of multi-dimensional PBEs. A mere change in connectivity of pivots in space, which changes their directionality, is shown to influence numerical results. This work led to new radial discretization of space, which has been followed up by others in the group and demonstrated to be quite powerful. A physical model is developed to understand digestive ripening of nanoparticles, a technique which is in extensive use in the literature to improve monodispersity of nanoparticles. The physical model is based on critical analysis of the large body of experimental findings available in the literature on several variations of this technique. The physical model is the first one to consistently and qualitatively explain all the reported experimental findings.

Chapter 1: Luminescence property of lanthanide and its applications Lanthanides are well-known for their unique luminescence property and have found widespread applications in sensing, bioimaging, lasers, optoelectronic devices, etc. Due to Laporte forbidden f-f transitions, lanthanides have very low intrinsic emission. The problem can be overcome by use of an ‘antenna’, which is an organic chromophore with excited state energy higher than the lanthanides’ emitting levels. Thereby it is possible to get highly emitting lanthanide complexes through energy transfer from the ‘antenna’. Due to long lifetime of lanthanides’ excited states, it's possible to perform time delayed measurement which is useful in bioassays and bioimaging since the short-lived background emission is effectively filtered. Research in supramolecular metallogels has grown rapidly in recent years, and already proven to have potential for designing advanced materials for a variety of applications, such as sensing, optoelectronics, catalysis, nanoparticle synthesis, biomedicine etc. A supramolecular gel where a lanthanide is an integrated part of it can combine the advantages of the supramolecular gel along with the unique property of lanthanide luminescence and thus such materials can be explored for potential applications. This chapter discusses the background information on the unique luminescence of lanthanides, and some examples of the applications of lanthanide complexes and lanthanide based gels. Chapter 2: Lanthanide luminescence based enzyme sensing in hydrogels This chapter describes the use of Tb/Eu luminescence in the sensing of biologically important enzymes. We discovered the sensitization of Eu(III) in Eu-cholate gel by 1-hydroxypyrene, and of Tb(III) in Tb-cholate gel by 2,3-dihydroxynaphthalene. These two sensitizers were covalently modified and sensitizer-appended hybrid (artificial) enzyme substrates were prepared for a few biologically important hydrolases. The covalently modified sensitizer termed as “pro-sensitizers”, didn't sensitize Tb(III)/Eu(III) in the hydrogel and no photoluminescence was observed. In the presence of the appropriate enzyme in the hydrogel, the pro-sensitizer was cleaved to liberate the sensitizer, which led to an enhancement of luminescence with time. Alkaline phosphatase and β-lactamase were assayed using pyrene phosphate and pyrene-oxo-cephalosporanic acid derivatives, respectively, in Eu-cholate hydrogel (Figure 1). β-Galactosidase was assayed using Tb(III) luminescence in Tb-cholate gel. The enzyme detection was based on red/green luminescence response from the gel. To understand the behaviour of the enzymes in the hydrogel, kinetic parameters were determined. The detection of different enzymes was also demonstrated in natural/biological samples like blood serum, milk and almond extract. Figure 1. Three different pro-sensitizers used for alkaline phosphatase, β-lactamase and β-galactosidase assays Chapter 3: Enzyme sensing on paper discs using lanthanide luminescence Developing a user-friendly biosensor is of considerable importance in clinical and analytical chemistry. Paper based biosensor design is an emerging field of research and paper based point of care (PoC) testing devices have already found applications in clinical, veterinary, environmental, food safety, security etc. Paper is made out of natural cellulose fibres, and has advantages of low cost, biodegradability, biocompatibility, and user friendliness. Paper based sensors have been used for the detection of ions, glucose, proteins, nucleic acids, antigens etc., with mostly colorimetric, fluorescent, electrochemical, chemiluminescence and Electrochemiluminescence readouts. In this work, the non luminescent Tb(III) and Eu(III) were embedded on paper as their cholate hydrogels and were used for detecting different hydrolases. Pro-sensitizers, as reported in Chapter 2, were immobilized on paper for the detection of a specific enzyme. The “pro-sensitizer” released the sensitizer upon enzyme action and led to luminescence enhancement from the gel coated paper disc. By this way, four different hydrolase enzymes detection were carried out using Tb(III)/Eu(III) luminescence as the readout (Figure 2) and the practical utility was demonstrated by the detection of specific enzymes in natural/biological samples. This paper disc based enzyme sensing provides a simpler and user friendly approach over the contemporary approach of enzyme sensing typically carried out in solution. Figure 2. Paper based biosensors for hydrolase enzymes Chapter 4: Luminescence resonance energy transfer in self-assembled supramolecular hydrogels Luminescence resonance energy transfer is a phenomenon of energy transfer between a FRET (Förster resonance energy transfer) pair, where a lanthanide is the donor. Lanthanides have attracted attention for the last several decades for their unique luminescence properties. LRET is a FRET process along with added advantages of Lanthanides, i.e. long lifetime of the lanthanides and characteristics emission spectra. LRET has been used for studying interaction of biomacromolecues, immunoassay, bioassays, etc. LRET in either a supramolecular organogel or a hydrogel is still an unexplored field. In this work we showed the energy transfer from Tb(III) to two different red emitting dyes in Tb-cholate hydrogel (Figure 3). The self assembly processes during hydrogelation assisted the energy transfer process without any need for laborious synthesis. The energy transfer was confirmed by time delayed emission, excitation spectra and lifetime measurement in the hydrogels. Energy transfer was observed both in the gel and the xerogel states. These luminescent materials may find applications in optoelectronics. Figure 3. Energy transfer from DHN to Tb3+ and then to red emitting dyes (Rhodamine B & Sulforhodamine 101) in the Tb-Cholate hydrogel Chapter 5: Room temperature synthesis of Lanthanide phosphate nanoparticle using a gel as a soft template Lanthanide orthophosphates are an important class of rare earth compounds, and have widespread applications in laser materials, optical sensors, heat resistance materials, solar cell etc. There are several methods in the literature for the synthesis of rare earth phosphate nanoparticles. Most of these are based on hydrothermal, microwave assisted, micro emulsion, arrested precipitation etc., which invariably dependent on stringent conditions such as (i) high temperatures and pressures, (ii) inert atmosphere and (iii) the use of external capping agents as stabilizers. Synthesis of such nanoparticles under milder conditions would always be preferable. In this context, the preparation of nanoparticles using hydrogel as template can be a possible alternative approach. The LnPO4 nanoparticle synthesis was done by diffusion of Na3PO4 in Ln-cholate hydrogels. The particles were characterized by transmission electron microscopy (TEM) and powder XRD analysis. TEM showed the formation of 3-4 nm size particles with an ordered arrangement on the gel fibre. This work demonstrated that the lanthanide cholate gels have high potential for the synthesis, and immobilization of lanthanide phosphate nanoparticles at room temperature to produce new types of composite materials. (For structural formula pl see the abstract pdf file)