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Correlation between DNA Self-Assembly Kinetics, Microstructure, and Thermal Properties of Tunable Highly Energetic AlCuO Nanocomposites for Micro-Pyrotechnic Applications Theo Calais, Aurélien Bancaud, Alain Esteve, and Carole Rossi ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00939 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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ACS Applied Nano Materials

Correlation between DNA Self-Assembly Kinetics, Microstructure, and Thermal Properties

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of Tunable Highly Energetic Al-CuO Nanocomposites for Micro-Pyrotechnic Applications

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Théo Calais †*, Aurélien Bancaud †, Alain Estève †, Carole Rossi †

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† LAAS-CNRS,

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The University of Toulouse, 7 Avenue du colonel Roche, F-31077 Toulouse, France

*Corresponding Author’s Present Contact Information: [email protected]. Singapore

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University of Technology and Design, 8 Somapah Road, Singapore 487372.

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KEYWORDS: DNA self-assembly; nanothermite; aggregation kinetics; nanocomposites; reaction-

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limited colloidal aggregation; diffusion-limited colloidal aggregation; exothermic reaction

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ABSTRACT

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The association of a metallic fuel (usually aluminum) with an oxidizer (metal oxide or organic

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compound) creates an exothermic material that can be ignited with an external stimulus such as

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local heating or spark discharge. These materials with high energetic performances, called

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nanothermites, have been used to release temperature or pressure waves for civil or military

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applications (initiators, impact igniters…). However, the energetic performances of these

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nanothermites are highly dependent on the nanoscale intimacy of the two components. The use of

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nanoparticles has been allowing an increase of the energy release, but the control of their assembly

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remains particularly challenging. In this work, we demonstrate that the use of DNA to self-organize

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Al or CuO nanoparticles greatly enhances the energy release of nanothermites by up to 240%

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compared to classically sonicated nanothermites in hexane, the heat of reaction prior to Al melting

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reaching a value of 2.57 kJ.g-1. In particular, we report that the energetic performances can be tuned

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by controlling the ionic strength during the self-assembly process. These results are supported by

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ultra-fine characterization of the nanocomposite microstructure based on high-resolution

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transmission electronic microscopy and energy-dispersive X-ray spectroscopy. Besides, we report

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the surprisingly good energetic performances of randomly mixed nanoparticles dispersed in water,

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nonetheless 40% lower than DNA self-assembled nanocomposites. Altogether, our study not only

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proposes an easy and immediate process for nanocomposites synthesis, but also opens the door for

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opportunities towards large-scale crystalline Al-CuO superlattices with high energetic

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performances.

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Over the two last decades, mixtures of metal and metallic oxides have been extensively studied

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for their exothermic properties. By reducing the size of components at the nanoscale, a large

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increase of the reactivity and the energy released by the oxido-reduction reaction occurring upon

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external stimulation has been detected.1,2 These mixtures, called nanothermites, are used for

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military applications (initiator, impact igniter)3,4, as well as for civilian purposes (automotive air-

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bag propellants, railroad welding,5 microthrusters)6. The metal of choice is aluminum because of

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the strong enthalpy of formation of its oxide form, alumina (Al2O3). A large variety of oxidizers

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(metallic as well as organic) can be associated with Al (CuO, Fe2O3, MoO3, Bi2O3, PTFE)2,7–9 to

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tune the energetic properties (combustion rate, ignition temperature or calorific power).

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However, the amount of energy released and the reactivity depends largely on the intimacy

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between the reducer and the oxidizer. The physical mixing of nanoparticles, e.g. by sonication of

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oxide and Al nanopowders in a volatile organic solvent, has become the predominant process to

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synthesize nanothermites, mainly due to the ease of process.9 However, this approach remains

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inadequate to control the assembly of nanoparticles at the nanoscale. Different self-assembly

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techniques have been developed to favor metal-oxide interactions and to limit metal-metal or

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oxide-oxide aggregations: the modification of nanoparticle surface charges by grafting opposite

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charged ligands or directly in aerosols,10–12 the use of graphene oxide sheets,13 or the grafting of

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proteins previously loaded with an oxidizer.14 The electrostatic interactions between oppositely

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charged metallic and oxide nanoparticles have been shown to enhance the burn rate of the

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assembled material.10-14 Another alternative explored by our group exploits the hybridization of

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two complementary DNA strands to drive the self-assembly of nanoparticles and precisely control

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the structure of the nanothermite.15 DNA has a double interest: (1) the accuracy and reversibility



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of the hybridization mechanism, and (2) the advances in DNA chemical modification that allow

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the addition of chemical cues, the control of inter-particle distance, etc…

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Indeed, due to its outstanding molecular recognition, DNA has become, over the last decades,

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an indispensable tool for nanotechnologies in a wide range of applications, including

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nanolithography,16 reversible soft materials,17 biological-based computation,18,19 high-ordered

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clusters for optical applications…20,21 Notably, DNA has been successfully used to self-assemble

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gold nanoparticles or quantum dots into large-scale crystalline superlattice structures22,23 with an

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accurate control of the properties: variation of the interparticle distances,24,25 modifications of the

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rigidity of the link between particles,26 and production of heterogeneous systems…21,27 These

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achievements have been made possible thanks to an extensive experimental and theoretical work

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on DNA functionalization of gold, silver, platinum or quantum dot nanoparticles,28–31, with a range

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of applications limited to medicine32,33 and plasmonic and opto-electronic devices.20,34

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Nanostructures based on other materials such as oxides or non-spherical particles are

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particularly challenging to synthesize with the same accuracy. Regarding Al and CuO

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nanoparticles, a proof of concept demonstrated the potential interest of DNA self-assembly,15 but

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the absence of a rigorous characterization of the functionalization process and the resulting

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microstructure did not allow a crystallization of nanoparticles into highly ordered structures. A

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substantial work has been devoted to characterizing the DNA grafting on these surfaces and to

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optimize its efficiency. Notably, interactions between DNA bases and oxide surfaces have been

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precisely characterized, revealing non-specific interactions and justifying the use of a

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body/antibody strategy to specifically graft DNA.35 Further DNA and streptavidin grafting

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densities and functionalities have recently been characterized and quantified,36 showing the poor


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coverage of streptavidin on nanoparticle surfaces and the subsequent poor hybridization efficiency.

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About 5 and 10% of grafted DNA strands on CuO and Al nanoparticles, respectively, could be

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hybridized (enough to ensure the self-assembly), while a large number of DNA strands were

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physisorbed on streptavidin-free areas on the nanoparticle surface, resulting in non-specific

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irreversible interactions during the self-assembly. The optimization of parameters during the

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functionalization process,36 and most substantially the design of the DNA sequence,37 enabled us

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to significantly reduce these non-specific interactions during the self-assembly. The impacts of

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these improvements on the resulting self-assembled nanostructure and its energetic properties have

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yet to be studied.

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In this context, this report presents the correlation between the self-assembly reaction with the

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resulting nano- and microstructures, and the ensuing thermal properties of DNA-functionalized

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energetic nanocomposite. The objectives of this work are twofold: (1) to demonstrate the DNA

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driven self-assembly of nanoparticles and, (2) to evaluate the impact of the self-assembly process

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conditions on the resulting microstructures and thermal properties. To achieve these goals, we

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studied self-assembly kinetics of Al and CuO nanoparticles by dynamic light scattering (DLS) and

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performed correlative analysis of the reaction product using high-resolution transmission electron

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microscopy (HR-TEM) coupled with scanning transmission microscopy (STEM). Finally, the

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analysis of the energetic properties of the synthesized nanothermites characterized by differential

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scanning calorimetry (DSC) demonstrated the impact of the nanostructuration on the properties of

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synthesized nanocomposites, allowing us to define the best processing conditions to maximize the

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energy released. We were able to tune the energy release by adapting the ionic strength of the

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solution during the self-assembly, from very weak energies (⁓0.6 kJ.g-1) to extremely high energies

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(⁓2.5 kJ.g-1). Finally (and surprisingly), nanocomposites randomly mixed in water (without DNA



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functionalization, initially prepared as a reference) exhibited an energy release, prior to Al melting,

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140% higher than those sonicated in hexane, suggesting an easy and immediate means to improve

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the nanothermite mixing process. On the other hand, a further optimization of DNA

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functionalization and the nanoparticle synthesis process could very soon lead to a large-scale

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crystalline superlattice structure with extremely high and controllable energetic properties.

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RESULTS AND DISCUSSION

We used the same DNA functionalization strategy for Al and CuO nanoparticles as reported in 36,37

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references

and summarized in Figure 1a. Details are given in the Methods section, and a

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summary of the colloids properties is provided in Table S1. Briefly, Al and CuO nanoparticles are

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firstly dispersed in stable aqueous colloidal suspensions. Streptavidin is added to the solution and

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non-grafted proteins removed via centrifugation after eight hours of incubation. Biotinylated-DNA

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strands are then added to the solution and non-grafted DNA strands are washed away via

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centrifugation. After functionalization, the two colloidal suspensions are mixed together in

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stoichiometric proportions, and the ionic concentration is increased to allow DNA hybridization by

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screening repulsive forces between negative particle surfaces.


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Figure 1. DNA self-assembly of Al and CuO nanoparticles into highly reactive Al-CuO

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nanothermites: schematic view of the (a) functionalization strategy and (b) multi-scale

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characterization of the self-assembled nanocomposites followed in this work. The DNA

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functionalization of nanoparticles is based on a 3-step process: (i) dispersion of nanoparticles in

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an aqueous solvent; (ii) streptavidin adsorption on nanoparticles surfaces; (iii) DNA grafting on

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streptavidin sites via biotin-streptavidin conjugation.

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Our strategy to characterize the self-assembled nanocomposites is schematized in Figure 1b.

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Aggregation kinetics are first analyzed as a function of salt concentration, and the resulting

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microstructures and chemical compositions are analyzed by HR-TEM and STEM analyses. Finally,

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thermal properties of nanocomposites are analyzed by DSC in the temperature range of ambient to



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700 °C. The impact of DNA functionalization and the self-assembly conditions are discussed by

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comparing four nanocomposites synthesized with different conditions: low salt concentration, high

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salt concentration, randomly sonicated in hexane, and randomly mixed in water.

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Aggregation kinetics of DNA-functionalized Al and CuO nanoparticles.

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The stability of colloidal suspensions of nanoparticles is generally ensured by repulsive

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Coulomb forces that stem from the surface charge of nanoparticles and the screen length related to

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the ionic strength of the solvent.38 The interaction potential of this force can be expressed as the

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sum of Van der Waals attractions (short range) and electrostatic repulsions (long range). It is

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necessary to reduce this barrier to initiate the aggregation of nanoparticles, either by reducing the

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charge at the nanoparticle surface or by increasing the ionic strength of the solution (to reduce the

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screen length). Both strategies have been previously employed by researchers to initiate the

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aggregation of gold nanoparticles.39–42 In our case, the grafting of DNA strands, which is associated

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to a high negative Zeta potential of ~-40 mV for Al and CuO nanoparticles (Table S1), does not

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allow an easy modification of the charge at the nanoparticle surface, justifying the choice of

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increasing the salt concentration.22,23,30

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In this study, aggregation kinetics were studied by monitoring the evolution of hydrodynamic diameter of aggregates as a function of time. The resulting data were fitted by a power law:42

𝐷 (𝑡) ∝ 𝐷 (0)𝑡


(1)

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with DH(t) the average hydrodynamic diameter of the aggregates at instant t, DH(0) the initial

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hydrodynamic diameter, and α the coefficient related to the fractal dimension df of the aggregate,

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as follows:

𝛼≡𝑧 𝑑

(2)

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The parameter z is related to the aggregation mechanism and the reactivity of clusters.42 Typically,

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when z < 1, the sticking probability of two small clusters is higher and the mean size of aggregates

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increases more slowly than for z ≥ 1. In the case of DNA-modified nanoparticles, the aggregation

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kinetics is controlled by the DNA hybridization reaction or the formation of irreversible and

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unspecific bonds.36 Two universal aggregation modes are commonly identified in literature,

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depending on the ionic strength:40

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(i)

reaction-limited colloid aggregation (RLCA) at low salt concentrations.

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(ii)

diffusion-limited colloid aggregation (DLCA) at high salt concentrations.

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In the RLCA regime, the aggregation is limited by the kinetics of the adhesion reaction. The

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formation of the aggregate is highly dependent on the ionic strength, its average size increases non-

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linearly over time and is associated to a higher fractal dimension df (~2.1). In DLCA regime, the

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aggregation is no longer limited by the fast adhesion reaction, but only by the diffusion of

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nanoparticles. The resulting kinetics is independent of the ionic strength and the resulting aggregate

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has a smaller fractal dimension df (~1.8), meaning a more dendritic conformation.



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In order to better assess the aggregation process involved for DNA-modified Al and CuO

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nanoparticles, kinetics was studied as a function of salt concentration (NaCl), ranging from 15 to

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250 mM, as reported in Figure 2a. Aggregation kinetics of non-functionalized Al and CuO

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nanoparticles (without DNA and streptavidin) were also studied as a reference and the kinetics

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curves are provided in Figure S1. The results were fitted by a power law as described in Equation

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(1), and the α coefficient is reported in Figure 2b. Note that the Zeta potential was also monitored,

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showing an increase from -40 to -15 mV with the increase of the salt concentration from 15 to 250

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mM (Figure S2).

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Figure 2. (a) Self-assembly kinetics of DNA-functionalized Al and CuO nanoparticles as a

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function of salt concentration. (b) Evolution of the coefficient inferred from power-law fitted

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kinetics curves of DNA functionalized Al and CuO nanoparticles (red points) and without DNA

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functionalization (black points). The dash lines were drawn to guide the eye.

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At the initial salt concentration (15 mM, black curve, Figure 2a), no aggregation is observed

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and the particles have a constant hydrodynamic diameter of ⁓250 nm over time. The aggregation

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kinetics is then accelerated by the increase of salt concentration, from 30 to 75 mM. Above 75 mM,


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the kinetics becomes roughly independent of the salt concentration with aggregates of 2 µm in

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hydrodynamic diameter after one hour. Note that above a certain size, aggregates sediment at the

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bottom of the reaction chamber. In the range of 15-50 mM, α increases from 0 to 0.48, and then

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slightly decreases and stabilizes at 0.42, in agreement with reported experimental values.41,42 Below

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50 mM, the aggregation kinetics is dependent on salt concentration, and aggregation is likely

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limited by the adhesion reaction: RLCA aggregate structures are expected to be dense and compact.

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Above 75 mM, the kinetics is independent of salt concentration, meaning that the aggregation is

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mainly limited by the diffusion of nanoparticles in the DLCA mode.

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The fractal dimension of the resulting nanocomposites was measured using the box-counting

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method on binarized SEM images, reported in Figure S3 and S4. The average df value obtained

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from eight different SEM images at different scales (see Methods section for details and Table S2

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for results) is 1.94 ± 0.03 and 1.77 ± 0.04 for RLCA and DLCA conditions respectively. These

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results are in good agreement with the reported fractal dimensions of ⁓2.1 and ⁓1.8, respectively.

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The slightly lower df value obtained in RLCA conditions could be attributed to the analysis of

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projected 2D images, while the one reported in literatures are obtained from light diffusion in 3D.

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Furthermore, the z factor can be estimated by using Equation (2), giving 0.90 and 0.75 for RLCA

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and DLCA conditions, respectively. The low z factor obtained in DLCA conditions, compared to

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the one observed for salt-induced gold nanoparticles aggregation (⁓0.85),40,42 suggests that the

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DNA hybridization reaction slows down the aggregation compared to electrostatic driven

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aggregations of the latter case, despite the diffusion-limited aggregation (confirmed by the non-

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dependence with salt concentration and the fractal dimension). Notably, the transition limit is

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shifted from ⁓50 mM to ~100 mM for colloids without DNA functionalization with a slightly



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higher plateau for α (⁓0.5, black curve, Figure 2b), confirming a faster aggregation purely caused

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by electrostatic adhesion reactions.

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Therefore, a clear limit between RLCA and DLCA modes seems to be identified at a NaCl

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concentration of ~50 mM for DNA-functionalized Al and CuO nanoparticles. Besides, control

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experiments were conducted to confirm that the aggregation is governed by DNA hybridization.

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First, the reversibility of the aggregation with temperature was checked using DLS. Secondly, the

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aggregation was stopped by saturating the solution with one type of DNA strands during the

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aggregation process. These control experiments are reported in Figure S5.

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Microstructure and chemical composition of self-assembled nanocomposites

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In order to clarify the structural arrangement of nanoparticles in the aggregates, we selected

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two DNA self-assembled nanocomposites synthesized with different ionic concentrations and

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compared their microstructure, chemical composition, and thermal properties. The low salt

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concentration is set to 35 mM (RLCA), because the contribution of electrostatic interactions of

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non-DNA functionalized surfaces at this value is negligible (Figure S1), and the high salt

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concentration is fixed at 250 mM (DLCA). The DNA self-assembled nanocomposites were

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respectively named DNA_Al-CuO_250mM and DNA_Al-CuO_35mM. Finally, we compared these

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nanocomposites with randomly mixed non-functionalized nanoparticles to characterize the

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influence of DNA. One was physically mixed in hexane (a commonly used protocol)9 and the other

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mixed in an aqueous solvent under the same ionic conditions as DNA_Al-CuO_35mM (without

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DNA functionalization). The composites were respectively named Al-CuO_hex and Al-

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CuO_NaCl-35mM. The aggregation kinetics curves of DNA_Al-CuO_35mM, DNA_Al-

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CuO_250mM, and Al-CuO_NaCl-35mM are merged in Figure S6 for an easier comparison. 12 ACS Paragon Plus Environment

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Notably, 35 mM is a much lower concentration than the one usually used for gold nanoparticles

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(100–300 mM).22,30

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TEM images of the four nanocomposites are shown in Figure 3. First, the poor mixing quality

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of Al-CuO_hex (randomly mixed in hexane, Figure 3a) results in big clusters of CuO nanoparticles

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(non-spherical dark particles), with an approximate surface area of 2 µm², that are surrounded by

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more dispersed smaller clusters of Al nanoparticles (spherical, light grey). Second, Al-CuO_NaCl-

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35mM (randomly mixed in water, Figure 3b) appears to be much thicker than in the other samples,

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as demonstrated by the difficulty to identify the nanoparticles in the cluster.

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Figure 3. TEM images of four nanocomposites: (a) Al-CuO_hex, randomly mixed in hexane;

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(b) Al-CuO_NaCl-35mM, randomly mixed in water; (c) DNA_Al-CuO_35mM, self-assembled in

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RLCA mode; (d) DNA_Al-CuO_250mM, self-assembled in DLCA mode. Scale bar is 0.5 µm.

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On the contrary, the nanoparticles in DNA_Al-CuO_35mM and DNA_Al-CuO_250mM

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(Figures 3c and 3d) can be easily visualized. Two different structures are discernable: an open

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structure for DNA_Al-CuO_250mM with a dendritic shape, and a denser and more compact

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structure for DNA_Al-CuO_35mM, confirmed at a wider scale by SEM images reported in Figure

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S3 and S4. In both cases, the alternative disposition of small CuO clusters (darker and smaller

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facetted nanoparticles around 300 µm²) with Al nanoparticles in a dense conformation reveals an

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excellent quality of mixing and intimacy between the nanoparticles, in comparison with Al-

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CuO_hex. Note that the local darker parts of the nanocomposite are due to a superposition of Al or

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CuO nanoparticles, but the thickness is much lower than Al-CuO_NaCl-35mM, suggesting a

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probable collapse of the structure during drying on the TEM grid. Additional TEM and SEM

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images at higher magnifications are shown in Figure S7 and S8.

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The chemical composition of Al and CuO nanoparticles can be further characterized by STEM

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technique with its sub-nanometer resolution and simultaneous chemical analysis with single-atom

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sensitivity, obtaining local information at the nanometer scale and providing an ultra-high chemical

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mapping resolution. Mapping of Al (blue), Cu (red) and Cl (green) elements is presented in Figure

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4. The corresponding dark-field STEM images are shown in Figure S9. The heterogeneity

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observed for Al-CuO_hex is confirmed in Figure 4a, where big red Cu blocks predominate. An

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analysis of a rectangular cross-section presented in Figure S10 shows that Al nanoparticles are

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disposed around 1-2 µm CuO blocks. For Al-CuO_NaCl-35mM (without DNA functionalization),


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Cl is detected all over the composite (in green, Figure 4Error! Reference source not found.b). This

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observation has been made in several positions and Energy Dispersive X-ray spectrum presented

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in Figure S11 confirm the presence of other “impurities”, due to the use of phosphate buffered

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saline (PBS) (see the Methods section), which are most probably trapped in the structure during

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the evaporation of the solvent and the subsequent salt crystallization. Therefore, the previously

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observed geometric features and the higher thickness can be explained by these crystals.

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Interestingly, the homogeneity of the nanocomposite is good, with Al and Cu clusters (of ~0.5 µm

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or less) regularly organized around NaCl crystals. The distribution of Al and Cu appears to be much

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more uniform for DNA_Al-CuO_35mM and DNA_Al-CuO_250mM, with small alternative clusters

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of Al or Cu (200-400 µm) (Figure 4c and 4d) compared to the bigger Cu aggregates of Al-

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CuO_hex (cf. cross-sections in Figure S10). Notably, no presence of Cl is detected in the DNA

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self-assembled composites.



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Figure 4. Chemical element mapping of Al (blue), Cu (red), and Cl (green) obtained by STEM

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analysis for (a) Al-CuO_hex, randomly mixed in hexane; (b) Al-CuO_NaCl-35mM, randomly

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mixed in water; (c) DNA_Al-CuO_35mM, DNA self-assembled in RLCA mode; (d) DNA_Al-

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CuO_250mM, DNA self-assembled in DLCA mode. Scale bar is 2 µm. Corresponding dark-field

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STEM images are reported in Figure S9.

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Finally, the homogeneity of the chemical compositions of the nanocomposites, i.e. the

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distribution of Al and Cu, enables us to gain a significant understanding of the role of DNA in the

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organization of nanoparticles at the nanoscale depending on the self-assembly conditions. Al, Cu,

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O, and Cl (the four most representative elements) were quantified by EDX-STEM measurements

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in three or more different positions on the TEM grid for the nanocomposites synthesized in a water-


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based solvent: DNA_Al-CuO_35mM, DNA_Al-CuO_250mM, and Al-CuO_NaCl-35mM. For each

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position, a quantification at four magnifications (×30k, ×50k, ×100k, ×150k) was performed to

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determine the dispersion of these elements at different scales. Results are reported in Table S3 and

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the calculated average results and standard deviations are reported in Figure 5.

Chemical composition (at.%)

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5

60

Al-CuO_NaCl-35mM DNA_Al-CuO_35mM DNA_Al-CuO_250mM

50 40 30 20 10 0

Al

Cu

O

Cl

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Figure 5. Atomic percentage composition of Al, Cu, O, and Cl elements for nanocomposites

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assembled in water: Al-CuO_NaCl-35mM, DNA_Al-CuO_35mM, and DNA_Al-CuO_250mM.

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Averaged results are obtained over at least 12 positions each, at four different scales (×30k, ×50k,

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×100k, ×150k). Original element percentages are reported in Table S3. Al-CuO_hex is not

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included in this study because of the fundamentally different synthesis method.

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First, DNA_Al-CuO_35mM and DNA_Al-CuO_250mM are composed of a similar proportion

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of Al, Cu, and O elements of about 50 at.%, 20 at.%, and 30 at.%, respectively. However, standard

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deviations are larger for DNA_Al-CuO_250mM (13%, 24%, and 12% for Al, Cu, and O elements,

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respectively) than for DNA_Al-CuO_35mM (8%, 12%, and 14% for Al, Cu and O elements,



1

respectively). This difference is likely explained by a higher proportion of randomly aggregated

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nanoparticles in high ionic concentrations due to the fast aggregation kinetics and the diffusion-

3

limited aggregation. The chemical composition of Al-CuO_NaCl-35mM is slightly different from

4

the other two nanocomposites, with a much higher proportion of O (40 ± 5 at.%) and Cl (12 ± 7

5

at.%), resulting in a lower proportion of Al (26 ± 8 at.%). These values are explained by the

6

presence of NaCl crystals and other pollutants all over the nanocomposite. Standard deviations are

7

much larger than the ones obtained for DNA_Al-CuO_35mM, confirming the random aggregation

8

of nanoparticles in water.

9

These results combined together help to understand the different mechanisms involved in the

10

assembly of Al-CuO_NaCl-35mM and DNA self-assembled nanocomposites. As shown in Figure

11

S6, we did not observe any aggregation for Al-CuO_NaCl-35mM, whereas the aggregates of

12

DNA_Al-CuO_35mM reach a hydrodynamic diameter of 1 µm after one hour. This suggests that

13

the aggregates visualized in SEM and TEM images takes place during the drying step, because the

14

evaporation of the solvent induces an increase of salt concentration which subsequently leads to

15

crystallization. We can assume that the nucleation of salt crystals is responsible for the aggregation

16

of nanoparticles around the nucleation sites, but further theoretical studies are required to confirm

17

this point. Nevertheless, the presence of Cl as a “bone structure” supports this hypothesis. On the

18

contrary, the absence of Cl in DNA self-assembled nanocomposites suggest that the aggregation is

19

mainly driven by DNA hybridization for DNA_Al-CuO_35mM and a combination of DNA

20

hybridization and electrostatic interactions for DNA_Al-CuO_250mM. Incidentally, the

21

crystallization of NaCl or phosphate salts during the evaporation of the solvent occurs, but outside

22

the composite, as demonstrated by eye-visible white crystals in the final product, notably for

23

DNA_Al-CuO_250mM.


Page 18 of 37

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1

Effect of DNA self-assembly on thermal properties

2

The energetic performances of the four nanocomposites are determined by DSC from 30 ºC to

3

700 °C to precisely characterize the effect of DNA functionalization and process conditions on the

4

low-temperature reactions before the melting of Al. The four curves are reported in Figure 6 and

5

present a similar profile: the onset temperature (temperature where the main exothermic reaction

6

starts) is ⁓490 °C and the main exothermic peak is centered at ~570 °C. The heat of reaction (ΔHexp)

7

is calculated by integrating DSC curves over time and normalizing the results with the sample mass

8

(from 100 °C to 620 °C for all experiments). Resulting ΔHexp are directly noted on the graphs.

9

Several weaker exothermic contributions can be observed in the temperature range of 200-500 °C

10

for nanocomposites assembled in water. These contributions could be attributed to the presence of

11

phosphate salts, as shown by EDX analyses, or other organic components like streptavidin and

12

DNA strands. The oxygen composition around CuO could be locally modified by these organic

13

compounds and induce a recrystallization of CuOx, as observed previously for Al-Cu-CuO

14

interfaces in this temperature range.43 Further experiments are required to identify these

15

contributions. Finally, the reactivity of the four nanocomposites, characterized by the slope of the

16

main peak, is similar in all cases.



5

DNA_Al-CuO_35mM DNA_Al-CuO_250mM

4 -1

Corrected heat flow (W.g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3 2 1

Page 20 of 37

exo.

-1

E35mM= 2.57 kJ.g

-1

E250mM= 0.61 kJ.g

0 5

Al-CuO_NaCl-35mM Al-CuO_hex

4 3 2 1

-1

Ewat = 1.82 kJ.g

Ehex = 0.76 kJ.g

-1

0 100

200

300

400

500

600

700

Temperature (°C)

1 2

Figure 6. DSC curves of nanocomposites heated from 30 °C to 700 °C at 10 °C/min under Ar.

3

DNA self-assembled nanocomposites DNA_Al-CuO_35mM and DNA_Al-CuO_250mM are

4

compared in the upper graph and randomly mixed nanocomposites Al-CuO_hex and Al-

5

CuO_NaCl-35mM are compared in the bottom graph. Heats of reaction are calculated by

6

integrating curves from 100 to 620 °C and noted on the graphs.

7

DNA self-assembled nanocomposites DNA_Al-CuO_35mM and DNA_Al-CuO_250mM are

8

compared in the upper graph. ΔHexp are 2.57 kJ.g-1 and 0.61 kJ.g-1 respectively. Contributions in

9

the temperature range of 200-450 °C are estimated to be around 25% of the total heat of reaction

10

measured for DNA_Al-CuO_35mM. No clear contribution is detected in this temperature range for

11

DNA_Al-CuO_250mM. To the best of our knowledge, the value obtained for DNA_Al-CuO_35mM

12

is the highest value obtained for an Al-CuO nanothermite. As a comparison, recent studies mixing


Page 21 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Al and CuO nanoparticles with graphene multilayers or ammonium perchlorate obtained an

2

optimized energy release of 1.68 kJ.g-1 and 1.11 kJ.g-1 respectively, for a similar range of

3

temperature.44,45 This higher performance is explained by a significant reduction of non-specific

4

interactions (due to the optimization of DNA sequences37 and functionalization processes36) and a

5

subsequent better efficiency of the DNA self-assembly, a denser structure (Figure 4), and a higher

6

homogeneity of the nanocomposite (Figure 5). Moreover, the presence of carbon in the DNA

7

structure could contribute to the oxidation of Al, as demonstrated for self-assembled nanothermites

8

on graphene oxide sheets.13,44 The homogeneity of DNA_Al-CuO_250mM is poorer and the

9

presence of refractory salt crystals after drying the nanocomposite inhibits the exothermic reaction.

10

At a smaller scale, the dendritic formation of the nanocomposite due to the DLCA regime has a

11

negative effect on the energetic properties due to void spaces and a poorer contact surface area

12

between the oxidant and the reducer.

13

In addition, the influence of DNA hybridization on the final properties of DNA self-assembled

14

energetic nanocomposites was evaluated by comparing the ΔHexp of DNA_Al-CuO_35mM with

15

nanocomposites obtained from Al and CuO nanoparticles functionalized with non-complementary

16

DNA strands. DSC curves are reported in Figure S12. Briefly, ΔHexp of nanocomposites assembled

17

with non-complementary DNA strands (where hybridization does not occur) decreases to 2.05 kJ.g-

18

1

19

Finally, it is known that a slow annealing of a DNA self-assembled nanostructure favors the

20

synthesis of a crystallized superlattice.30 A similar experiment was conducted with our colloids but

21

neither the ΔHexp nor the homogeneity of the chemical composition was clearly improved (not

22

shown). Indeed, an increase of the temperature also reduces the energetic barrier that ensures the

23

stability of colloidal suspensions, which can probably lead to an increase of the proportion of non-

. This result reveals the impact of DNA on the structuration of the nanocomposite at the nanoscale.



1

specific interactions, especially regarding the sensitivity of our colloids to salt concentration. In

2

addition, a preliminary study of DNA self-assembled nanocomposites obtained at higher salt

3

concentrations confirmed the maximization of the energy release at 35 mM (data not shown).

4

However, it is difficult to elucidate the significance of microstructural features of the self-

5

assembled nanocomposites in the NaCl concentration range of 50–100 mM due to the multiplicity

6

of mechanisms involved in the aggregation process (mixing of DLCA and RLCA mechanisms,

7

increase of non-specific interactions); this requires a more substantial analysis.

8

Al-CuO_hex and Al-CuO_NaCl-35mM, are compared in the bottom graph and the ΔHexp are

9

respectively 0.76 kJ.g-1 and 1.82 kJ.g-1. The first value is closer to other published data,9,46 while

10

the ΔHexp of Al-CuO_NaCl-35mM is much higher, despite a random nanoparticle assembly. An

11

extra exothermic peak is observed for Al-CuO_NaCl-35mM, centered at 670 ºC. This peak occurs

12

beyond the melting temperature of Al (~660 ºC) and is most probably caused by the reaction of the

13

remaining melted Al with Cu2O and CuO, as previously observed.44,45 Interestingly, this peak was

14

observed for several other similar samples prepared in aqueous media without DNA. The reason

15

why this contribution is not observed for DNA self-assembled nanocomposites is not clear and

16

requires complementary analyses at higher temperatures. This contribution is not considered in the

17

calculated energy (only the temperature range from 100 ºC to 620 ºC is considered for all samples).

18

The exothermic contributions in the temperature range of 200-450 °C are estimated to be 20% of

19

the measured heat of reaction and do not solely explain this surprising result. However, the better

20

intimacy between Al and CuO nanoparticles demonstrated by chemical element mappings and

21

quantifications (Figure 4 and 5) does explain this increase. The main difference with the sonication

22

of nanoparticles in hexane remains in the mixing of individually stabilized aggregates (~250 nm)

23

in aqueous solvent, while Al and CuO nanopowders (with much bigger aggregates usually found


Page 22 of 37

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1

in commercial nanopowders) are sonicated and mixed and sonicated at the same time in hexane.

2

Other elements that contribute to the better performance are the denser structure, which induces a

3

better intimacy between components, and the presence of phosphate salts, which are rich in oxygen

4

and have an oxidizing property. Recently, a study pointed out the probable impact of Cl- ions in

5

the removal or dissolution of the alumina layer during the oxidation of the Al core of Al-CuO

6

nanoparticles prepared with ammonium perchlorate, enhancing the energy release from 792.1 J.g-

7

1

8

nanothermites assembled in an aqueous solvent have a much better mixing intimacy, which resulted

9

in better performances and a higher heat of reaction than nanothermites processed in hexane.

10

Moreover, the use of an aqueous solvent, as opposed to volatile and flammable organic solvents,

11

also reduces the risk of accidental ignition of the nanothermite.47 Therefore, replacing the classic

12

physical mixing process by a two-step preparation in an aqueous solvent could significantly

13

improve the energetic performances of nanothermites.

14 15

to 1113 J.g-1.45 This phenomenon could also occur in our nanothermites. We concluded that

Table 1. Variations of the heat of reaction calculated in the range of 100-620 °C of nanocomposites normalized with Al-CuO_hex and Al-CuO_NaCl-35mM (%).

Al-CuO_NaCl-

DNA_Al-

DNA_Al-

35mM

CuO_35mM

CuO_250mM

X

140%

240%

-20%

-60%

X

40%

-65%

Reference:

Al-CuO_hex

Al-CuO_hex Al-CuO_NaCl35mM

16

To summarize the impact of the aggregation strategy on the material energetic performances,

17

results are normalized using Al-CuO_hex and Al-CuO_NaCl-35mM as a reference, and reported in

18

Table 1. The DNA self-assembly strategy, applied in a low ionic strength solution, induces an



1

increase of 240% in the heat of reaction released prior to Al melting (660 °C) with Al-CuO_hex as

2

a reference. With high ionic strength conditions, the heat of reaction released prior to Al melting

3

decreases by 20%. Surprisingly, the heat of reaction of Al-CuO_NaCl-35mM is also increased by

4

140%. Consequently, the use of a DNA self-assembly strategy implies an increase of 40%

5

compared to the randomly assembled Al-CuO_NaCl-35mM, and leads to the highest heat of

6

reaction recorded so far for an Al-CuO nanothermite.

7

CONCLUSIONS

8

In this work, we demonstrated the impact of DNA self-assembly on the synthesis of highly

9

reactive Al-CuO nanocomposites. The use of low salt concentration solutions for the DNA self-

10

assembly of nanoparticles results in a highly energetic nanomaterial with a dense microstructure

11

and an excellent homogeneous mixing of Al and CuO nanoparticles, as inferred from HR-TEM

12

and STEM analyses. The heat of reaction released prior to Al melting is 240% higher compared to

13

a nanothermite synthesized by physical mixing in an organic solvent, reaching 2.6 kJ.g-1. The

14

increase of 40% of the heat of reaction compared to our first publication underlines the significant

15

enhancement of the DNA functionalization of Al and CuO nanoparticles and their self-assembly.

16

The dependence of self-assembly kinetics on ionic concentrations, interpreted using colloid

17

aggregation sciences, enables us to tune the microstructure of the self-assembled nanocomposites

18

and the resulting energetic performances. A slow aggregation in a reaction-limited regime allows

19

for a better organization of nanoparticles at the nanoscale, while a diffusion-limited aggregation

20

significantly decreases energetic properties (dropped by 260%), because of a less homogeneous

21

material at the nanoscale, the presence of refractory NaCl crystals, and a dendritic microstructure.


Page 24 of 37

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1

Finally, we experimentally demonstrated that the aggregation of Al and CuO nanoparticles in

2

water with identical conditions as DNA-functionalized nanoparticles could increase the heat of

3

reaction by 140% compared to physically mixed nanothermites in hexane. This is explained by a

4

better mixing intimacy for water-processed nanothermites. The use of DNA is still justified by the

5

40% increase of the heat of reaction compared to water-processed nanothermites, but further

6

developments could lead to a superlattice crystallization by reducing non-specific interactions.

7

Notably, the coverage of functional DNA strands could be significantly increased (only ~20% of

8

the surface is covered by functional DNA strands),36 helping the stabilization of colloidal

9

suspensions. Moreover, the use of laboratory-made Al or CuO nanoparticles could be a significant

10

step in reducing the component size and shape distributions with the functionalization of individual

11

particles. With these improvements, we believe that the crystallization of Al-CuO superlattices is

12

attainable, and that nanothermites are likely to broaden their field of applications thanks to a high

13

level of control of their microstructure and chemical functionality.

14

Although the use of DNA nanotechnologies significantly increases the cost of production of

15

self-assembled nanothermites (in both time and economic aspects), the promise of an optimized

16

tunable multifunctional nanothermite can satisfy highly-advanced technologies for niche sectors

17

like initiators for military and aerospace applications. These applications do not require a mass

18

production but are highly demanding in efficiency and multifunctionalities, contrary to other

19

applications like initiators for airbag security systems. For these mass applications, the mixing of

20

colloidal suspensions in aqueous solvents, as shown in this article, is an excellent alternative to the

21

more expensive DNA self-assembly process and the much less efficient sonication in hexane. To

22

conclude, this article presents two different approaches for the synthesis of optimized

23

nanothermites, which are both able to satisfy different applications in micro-pyrotechnics.



1

METHODS

2

Reagents.

3

NaCl powder, monobasic and dibasic sodium NaH2PO4 and NaHPO4- powders, PBS 10X, and

4

surfactant Tween 20 were purchased from Sigma-Aldrich. Phosphate buffer (denoted PB, 0.2 M)

5

at pH 7 was obtained by mixing monobasic and dibasic powders (39% vol. and 61% vol.,

6

respectively). NaCl solutions (2 M) were prepared by dissolution of NaCl powder in ultra-pure

7

water. Dried streptavidin protein was purchased from Sigma-Aldrich and diluted in PBS 1X at a

8

final concentration of 1 mg.L-1. Sequences of oligonucleotides were designed as detailed in ref 37

9

with a spacer of 15 repeated thymidines (denoted -(T)15) and reported in Table 2. Biotinylated

10

oligonucleotides were purchased from Eurogentec in dried form and then diluted in ultra-pure

11

water at a final concentration of 0.5 mM. The spacer is made of 15 repeated thymines.

12

Table 2. Sequences of Oligonucleotides used in this work.

Name

Sequence (5’ to 3’)

ss-A15

Biotin-(T)15 -ACA-TCG-CCC-CGC-CT-6FAM

ss-B15

Biotin-(T)15-AGG-CGG-GGC-GAT-GT-6FAM

13

CuO nanopowders (50 nm, facetted nanoparticles) were purchased from Sigma-Aldrich. Al

14

nanopowders (70 nm, spherical nanoparticles) coated with an alumina shell were purchased from

15

US Research Nanomaterials (Austin, TX). Size dispersion was characterized for both commercial

16

nanopowders by SEM imaging.36


Page 26 of 37

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1

Preparation of DNA-functionalized Al and CuO nanoparticles.

2

Al (14.5 mg) and CuO (13 mg) nanopowders were separately suspended in an aqueous solution

3

composed of 10 mM PB (pH = 7.3, limiting nanoparticle oxidation)48 and 0.05% (vol:vol) Tween

4

20 in ultra-pure deionized water. Tween 20 was chosen as a surfactant to increase nanoparticles

5

stability and for its neutrality regarding biological interactions.15 The nanoparticles were then

6

placed in a shaker and sonicated for 8 min. The mass of the Al and CuO powders, the composition

7

of the solution and the sonication time were optimized using a Doehlert experiment design method

8

to minimize hydrodynamic diameters. The nanoparticle concentration was then determined from

9

the hydrodynamic diameter of nanoparticles measured by DLS as well as Cu and Al mass

10

concentrations measured by Atomic Absorption Spectroscopy (AAS) and maximized by varying

11

buffer and surfactant concentrations, as previously described.36

12

After sedimentation time of 24 h, 20 mL of the supernatant of each colloidal suspension were

13

resuspended in a centrifuge tube. After the addition of 30 µL of streptavidin (1 mg/mL), the

14

solutions were homogenized by vortexing for 10 s and then sonicated for 5 min. After an incubation

15

period of 24 h, the colloidal suspensions were rinsed with aqueous buffer (PBS 0.1X, 0.05% vol:vol

16

Tween 20 and pure water) to remove excess protein.

17

Finally, 10 µL of biotinylated DNA at a concentration of 0.5 mM was added to the colloidal

18

suspensions. After an incubation time of 24 h, the solutions were similarly rinsed with an aqueous

19

buffer (PBS 0.1X, 0.05% vol:vol Tween 20 and pure water) to remove excess DNA.

20 21

Hydrodynamic diameters and Zeta potentials of each Al and CuO suspension before and after biological species functionalization are reported in Table S1.



1

Synthesis of Al-CuO nanothermites.

2

The physically mixed Al-CuO nanocomposite, named Al-CuO_hex, were prepared using a

3

classical sonication procedure.9 Al and CuO nanopowders were sonicated in hexane solution for a

4

total of 4.5 min with a 1 s delay after each 2 s of sonication to avoid the heating of the solution,

5

which could lead to undesired ignition. Mass of Al and CuO powders were calculated to obtain a

6

1:1 ratio of Al and CuO nanoparticles (same conditions as nanothermites synthesized in water).

7

The hexane is then evaporated at 70 °C under 2  104 Pa. The powders were then characterized by

8

HR-TEM and considered ready for DSC experimentations.

9

The nanocomposites randomly aggregated in water, named Al-CuO_NaCl-35mM, were

10

prepared by mixing Al and CuO colloidal suspensions (before streptavidin functionalization) in

11

stoichiometric proportions. Here, stoichiometric ratio means an equal number of Al and CuO

12

nanoparticles in the solution. Regarding the nanoparticle concentrations determined by AAS, a

13

typical volume ratio of 2.1 between CuO and Al colloids is respected.36 After mixing, the NaCl

14

concentration is increased to 35 mM (same conditions as DNA_Al-CuO_35mM). Aggregation

15

kinetics is then monitored by DLS. After a sedimentation time of two or three days, the supernatant

16

is progressively eliminated despite a poor sedimentation. The last milliliters of solvent are dried in

17

an oven at 40 °C under 2  104 Pa for 4 h. After drying, the powder was characterized by HR-TEM

18

and DSC experiments.

19

DNA self-assembled nanocomposites were prepared by mixing DNA-functionalized Al and

20

CuO colloidal suspensions, in stoichiometric proportions with a typical volume ratio of 2.1 between

21

CuO and Al suspensions. Aggregation was then initiated by increasing the NaCl concentration


Page 28 of 37

Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

from 15 to 250 mM and monitored by DLS. The NaCl concentration was set to 35 mM and 250

2

mM for the synthesis of DNA_Al-CuO_35mM and DNA_Al-CuO_250mM, respectively. After

3

sedimentation (usually one day), supernatant discarding, and elimination of last traces of solvent

4

in an oven at 40 °C under 2  104 Pa for 4 h, the resulting DNA self-assembled nanocomposites

5

were analyzed by HR-TEM and DSC experiments. Two control experiments reported in Figure

6

S5 confirmed the reversibility of the aggregation and the functionality of colloids.

7

Nanoparticle characterization.

8

A Zetasizer Nano ZS instrument (Malvern Instruments) was used to determine the nanoparticle

9

hydrodynamic diameters by diffraction light scattering (DLS) and zeta potentials were determined

10

by Doppler laser electrophoresis. The signal of a He/Ne laser emitting at 632.8 nm was deviated

11

by a measurement cell of 150 µL or 850 µL for hydrodynamic or zeta potential analyses,

12

respectively. All zeta potential and hydrodynamic diameter measurements were performed at 25 °C

13

and at pH 7.3.

14

Scanning Electron Microscopy (SEM) imaging of nanocomposites was performed using a

15

Hitachi S4800 instrument coupled with an EDX Spectroscopy system. High-Resolution

16

Transmission Electron Microscopy (HR-TEM) imaging of nanocomposites was performed with a

17

JEOL JEM-ARM-200F cold FEG instrument equipped with an EDX spectrometer. The samples

18

were prepared by depositing and evaporating a droplet of the aqueous colloidal solution on a

19

carbon-coated nickel grid.

20

Fractal dimensions of aggregates were calculated from binarized SEM images re-sized to 1024

21

× 1024 pixels, using a box counting method on Matlab software. The image is iteratively divided



1

by a 2i scale factor for i varying from 1 to 10. Then, black pixels are counted and df is calculated

2

from the linear regression of the log-log graphic representation of the number of black pixels as a

3

function of the scale factor. For both RLCA and DLCA conditions, eight SEM images are analyzed

4

at different magnifications: one image at ×5k, three at ×10k, three at ×25k and one at ×5k. Results

5

reported in the article are obtained after averaging the df for the eight images. One example of SEM

6

image for each magnification and all calculated df are reported in Figure S4 and Table S2

7

respectively.

8

Differential scanning measurements were performed with a Mettler-Toledo calorimeter

9

equipped with a HSS8 sensor. The temperature range for all experiments was 30–700 °C with a

10

heating rate of 10 °C.min-1. The flowing gas is 99.999% Ar purified by passing through an oxygen

11

trap (Supelco). The sample was deposited into a platinum crucible and weighed before the

12

experiment (1-5 mg). After the first heating cycle, the sample is cooled down to room temperature

13

and then heated again twice at the same heating rate. The second analysis is used to correct the

14

baseline and the third one to verify that the bulk heat capacity of the sample did not change between

15

the first and the second ramps.

16

ASSOCIATED CONTENT

17

Supporting Information

18

Physico-chemical properties of Al and Cuo colloidal suspension, calculated fractal dimensions,

19

aggregation kinetics of non-functionalized Al and CuO nanoparticles as a function of salt

20

concentration, Zeta potential evolution as a function of salt concentration, SEM images used for

21

the fractal dimension calculation, reversibility of the assembly with temperature, aggregation


Page 30 of 37

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

kinetics

2

complementary SEM and TEM images of DNA self-assembled nanocomposites, chemical element

3

distributions along a cross-section of nanocomposites, comparison of EDX spectra of Al-

4

CuO_NaCl-35mM and DNA_Al-CuO_35mM, comparison of DSC curves of DNA self-assembly

5

nanocomposites and composites synthesized from nanoparticles functionalized with non-

6

complementary strands.

7

AUTHOR INFORMATION

8

Corresponding Author

9

of

DNA_Al-CuO_35mM,

DNA_Al-CuO_250mM

*Email: [email protected]

10

Current address:

11 12 13 14

Digital Manufacturing and Design Center (DmanD) Singapore University of Technology and Design 8 Somapah Road Singapore 487372

15 16

17

18

Notes Conflict-of-Interest Disclosure

The authors declare no competing financial support.

Funding Sources


and

Al-CuO_NaCl-35mM,


Page 32 of 37

1

This study is supported by IDEX – University of Toulouse grant (ref. 138241) and French

2

National Defense Agency.

3

ACKNOWLEDGMENT

4

The

authors

would

like

to

thank

Teresa

Hungria-Hernandez

from

“Centre

de

5

MicroCaractérisation Raymond Castaing (UMS 3623)” for her great support with TEM

6

experiments.

7

REFERENCES

8 9

(1)

Pantoya, M. L.; Granier, J. J. Combustion Behavior of Highly Energetic Thermites: Nano versus Micron Composites. Prop. Explos. Pyrot. 2005, 30, 53–62.

10 11 12

(2)

Weismiller, M. R.; Malchi, J. Y.; Lee, J. G.; Yetter, R. A.; Foley, T. J. Effects of Fuel and Oxidizer Particle Dimensions on the Propagation of Aluminum Containing Thermites. P. Combust. Inst. 2011, 33, 1989–1996.

13 14 15

(3)

Taton, G.; Lagrange, D.; Conedera, V.; Renaud, L.; Rossi, C. Micro-Chip Initiator Realized by Integrating Al/CuO Multilayer Nanothermite on Polymeric Membrane. J. Micromech. Microeng. 2013, 23, 105009.

16 17 18

(4)

Glavier, L.; Nicollet, A.; Jouot, F.; Martin, B.; Barberon, J.; Renaud, L.; Rossi, C. Nanothermite/RDX-Based Miniature Device for Impact Ignition of High Explosives. Propell. Explos. Pyrot. 2017, 42, 308-317.

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(5)

Apperson, S.; Shende, R. V.; Subramanian, S.; Tappmeyer, D.; Gangopadhyay, S.; Chen, Z.; Gangopadhyay, K.; Redner, P.; Nicholich, S.; Kapoor, D. Generation of Fast Propagating Combustion and Shock Waves with Copper Oxide/Aluminum Nanothermite Composites. Appl. Phys. Lett 2007, 91, 243109.

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(6)

Apperson, S. J.; Bezmelnitsyn, A. V.; Thiruvengadathan, R.; Gangopadhyay, K.; Gangopadhyay, S.; Balas, W. A.; Anderson, P. E.; Nicolich, S. M. Characterization of Nanothermite Material for Solid-Fuel Microthruster Applications. J. Propul. Power 2009, 25, 1086–1091.


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