(542b) Parametric Study on Gold Nanoparticles Flow Synthesis in a Microwave-Assisted Reactor

Gold nanoparticles (AuNPs) present unique electronic, optical and chemical properties which allow them to be used in various sectors such as electronics, catalysis, sensors, biotechnology and biomedical applications. Of key importance in the synthesis is the control of size, shape and particle size distribution (PSD). Narrow PSD of specific shape and size ensures realising the desired properties for the intended application.^[1] Microwave heating has been used for nanoparticle synthesis in both batch and continuous flow reactors. The major advantage of microwave over conventional heating is the rapid volumetric and selective heating of the medium. Such heating mode can mediate particle formation on the inner reactor walls and lead to a â??greenâ? technology due to low energy losses.^[2]

The current study addresses the synthesis of gold nanoparticles in a flow reactor in a single-mode microwave applicator. Nanoparticles were synthesised using the citrate reduction method (Turkevich) and experiments showed repeatable results. The parameters studied were (i) the applied microwave power, (ii) the citrate/gold ratio and (iii) the flow rate of the synthesis mixture. UV-vis absorption spectra of the samples were taken immediately after collection from the reactor (stage I) and in regular intervals, until the surface plasmon resonance (SPR) band was stable (stage II). Analysis of the samples in stage II showed that increasing microwave power decreased polydispersity of synthesised particles, while SPR bands of the particles were in the range 525-535nm and final particle size was 20-30nm. The aspect ratio of the formed particles decreased with time from 1.5 (stage I) to 1.2 (stage II), indicating particles shifting from ellipsoidal to spherical shape.

Increasing the citrate/gold ratio of the inlet mixture from 1/1 to 60/1, while keeping constant flow rate, decreased the final size and dispersity of synthesized particles in stage II, from ~68 to 10nm, as citrate acts both as reducing and stabilizing agent.^[3] Interestingly, stage I samples showed a minimum in particle size at citrate/gold ratio ~10. Increasing the flow rate of the synthesis solution from 4 to 10ml/min resulted in lower outlet reactor temperature, as microwave power is volumetric and affected by the flow rate of the medium.^[4] Temperature affected the nucleation and growth stage of nanoparticle synthesis and UV-vis absorption and differential centrifugal sedimentation (DCS) analysis of the samples in stage II showed that increasing the flow rate resulted in larger and polydispersed PSD. A comparative analysis was conducted between control samples and samples in stage II. Control samples were prepared in batch by mixing citrate and gold precursor and letting them react at room temperature, until the surface plasmon resonance (SPR) band was stable. For the same citrate/gold ratio, control samples showed larger, more disperse AuNPs than samples prepared under microwave power in stage II.

References

^[1] Seol, S.K., et al., Microwave synthesis of gold nanoparticles: Effect of applied microwave power and solution pH. Materials Chemistry and Physics, 2011. 131(1-2): p. 331-335.

^[2] Sturm, G.S.J., et al., Microwaves and microreactors: Design challenges and remedies. Chemical Engineering Journal, 2014. 243: p. 147-158.

^[3] Kumar, S., K.S. Gandhi, and R. Kumar, Modeling of Formation of Gold Nanoparticles by Citrate Method. Industrial & Engineering Chemistry Research, 2007. 46(10): p. 3128-3136.

^[4] Meredith, R., Engineersâ?? Handbook of Industrial Microwave Heating. The Institution of Engineering and Technology, 1998.