Microfluidic devices and green protocols for the synthesis of time-stable copper nanoparticles

LAURA GUTIERREZ; ESTEBAN GIORIA

Venecia

Otro; Green Chemistry Postgraduate Summer School; 2020

IUPAC: International Union of Pure and Applied Chemistry

Metallic nanoparticles are being employed for many applications such as bio-sensing, electronics, drug delivery, catalysis, among others. Particularly, copper nanoparticles (CuNP) are of great interest as a non-expensive alternative to traditional novel metal like Pt and Pd. CuNP have been applied in oxidation reactions, reduction of water pollutants, selective catalytic reduction of NO, as an antimicrobial nanomaterial, in the reverse water gas shift reaction, among others. However, CuNPs tends to agglomerate and oxidize easily under contact with air. Thus, changes in their physicochemical properties have a negative impact on its applications. Controlling the synthesis conditions is a key factor to obtain small and time-stable nanoparticles. Therefore, most of the reported protocols involve the use of toxic and concentrated reducing and capping agents like NaBH4 and CTAB or PVP, respectively. However, the transition to a green chemistry, employing new and optimized processes with less toxic reagents, is of major relevance. In this work, we employ starch as a sustainable, biodegradable, cheap, non-toxic and green capping agent for the CuNP stabilization. Also, and in order to compare both set-up, the synthesis was carried out in a continuous microreactor (CuNP-M) and in a conventional batch reactor (CuNP-B). The use of microfluidic devices allows to enhance the mass and energy transfer during the reaction, operating under safer conditions.For the synthesis, copper acetate was employed as metallic precursor and an aqueous hydrazine solution as reducing agent. The use of N2H4 allows to carry out the reaction at room temperature, fixing only 1 minute of residence time and forming N2 as a non-toxic by-product.The formation of CuNP was verified by UV-vis spectroscopy, where the LSPR band between 585 and 620 nm corresponding to nanometric metallic copper nanoparticles could be observed for both set-up. The mean particle size was determined by Dinamic Light Scattering, indicating values of 10 nm and 13 nm for CuNP-B and CuNP-M, respectively. Following the stability by DLS, the mean particle size for CuNP-B changes to 150 nm after 14 days, indicative of an agglomeration process. However, the particle size for CuNP-M did not substantially change. When using the microreactor, mass and heat transfer processes are more effective than in the conventional batch. The micro sized reaction volume tends to an optimal mixture of the reagents, whit enhanced control of the nucleation, growth and stabilization of the nanoparticles.On the one hand, XRD measurements indicates that in the batch round flask reactor, not only Cu was formed but also CuO and Cu2O were detected. On the other hand, the microreactor device formed mainly metallic copper nanoparticles. Only a slight diffraction peak of CuO was detected. This could be related that in the microreactor, the contact air is avoided and the highly reducing conditions preserve the CuNP against oxidation. In conclusion, we demonstrated that CuNP can be obtained even at room temperature employing microreactors and environmentally friendly protocols. The use of microfluidic devices allows to enhance the reaction conditions, forming time-stable and non-oxidized nanoparticles, following the remarks of the green chemistry principles.