Efficient quenching sheds light on early stages of gold nanoparticle formation

Abstract

The formation mechanism of plasmonic gold nanoparticles (Au NPs) by fast NaBH₄ induced reduction of the precursors is still under debate. In this work we introduce a simple method to access intermediate species of Au NPs by quenching the solid formation process at desired time periods. In this way, we take advantage of the covalent binding of glutathione on Au NPs to stop their growth. By applying a plethora of precise particle characterization techniques, we shed new light on the early stages of particle formation. The results of in situ UV/vis measurements, ex situ sedimentation coefficient analysis by analytical ultracentrifugation, size exclusion high performance liquid chromatography, electrospray ionization mass spectrometry supported by mobility classification and scanning transmission electron microscopy suggest an initial rapid formation of small non-plasmonic Au clusters with Au₁₀ as the main species followed by their growth to plasmonic Au NPs by agglomeration. The fast reduction of gold salts by NaBH₄ depends on mixing which is hard to control during the scale-up of batch processes. Thus, we transferred the Au NP synthesis to a continuous flow process with improved mixing. We observed that the mean volume particle sizes and the width of the particle size distribution decrease with increasing flow rate and thus higher energy input. Mixing- and reaction-controlled regimes are identified.

Fig. 1. (a) Scheme of continuous flow synthesis; (b) scheme of the in situ fiber UV-vis spectrometer, (i) light source, (ii) fiber optics, (iii) housing, (iv) cuvette holder on a magnetic stirrer, (v) PC for data recording and processing, (c) scheme of quenching the Au NP growth by adding GSH and measurement of the PSDs of the quenched samples by HPLC-SEC by using a calibration curve for assigning the measured retention time distribution to a PSD.

Fig. 2. (a) Comparison of PSDs measured by AUC and HR-STEM from batch-synthesized Au NPs. (b) Representative HR-STEM micrograph of spherical batch synthesized Au NPs.

Fig. 3. (a) Extinction at 510 nm over time. Different kinetic regimes are visible which start from a rapid nucleation and slow down to a much slower growth (left axis). The black and blue data points belong to two different quenching experiments “V1” and “V2”, performed under identical conditions to check the reliability of the quenching method. On the right axis, the median core diameter is shown as measured by HPLC-SEC (yellow data points). The particle number concentration decreases from 6.1 × 10²⁰ m⁻³ in the beginning to 3.0 × 10¹⁹ m⁻³ at the end. (b) In situ UV/vis spectra as a function of quenching time t_q. The spectrum at 0 s refers to a preliminary mixture of precursor solution and GSH before the reduction agent solution was added.

Fig. 4. Comparison of HPLC-SEC and AUC results for GSH-Au NP samples. Normalized PSDs of quenched Au NP samples measured by HPLC-SEC are shown for (a) hydrodynamic diameter and (b) Au core diameter. (c) UV/vis spectra of different species in a GSH-Au NP sample quenched after 2 s as retrieved by HPLC-SEC. The components of the sample exhibit retention times t_R between 21 min and 23 min. (d) Sedimentation coefficient distributions of GSH-Au NP samples quenched between 0 s and 40 s measured by SV-AUC.

Fig. 5. Experimentally obtained and modelled values for the temporal evolution of the median value for the Au NP core diameter (right axis) and of the relative Au NP number concentration (left axis). (a) Total time range. (b) Time range from 0–70 s enlarged for better visualization of the early stages. The particle number concentration at the beginning N₀ amounts to 6.1 × 10²⁰ m⁻³.

Fig. 6. Proposed formation mechanism of small spherical plasmonic Au NPs by fast reduction of Au(iii) ions by the strong reducing agent NaBH₄. As first semiconducting Au clusters are formed due to supersaturation of Au atoms. The Au clusters grow by agglomeration to plasmonic Au NPs.

Fig. 7. (a) UV/vis spectra and (b) PSDs q₃(x) of Au NPs synthesized in a T-mixer at increasing Reynolds number as measured by AUC.

Fig. 8. Modal particle size x_mod,3 of the q₃(x) measured by AUC over the respective Reynolds number (Re) (left axis) and standard deviation of the lognormal PSDs (right axis).