Colloidal stability of the coated gold nanoparticles synthesized from spent coffee ground extract and qualitative modeling of nucleation and growth of gold nanoparticles

Gold nanoparticles (AuNPs) were synthesized using cold-brew spent coffee grounds extract. The synthesized AuNPs with two average sizes, using 1.8 mM HAuCl4 (1.8AuNPs and 4.0 mM HAuCl4 (4.0AuNPs), were compared for their enhancement of colloidal stability to withstand several conditions, including mechanical, chemical, thermal, and acid-base disturbances. Furthermore, the synthesized AuNPs were coated with polyethylene glycol 400 and 6000 (PEG400 and PEG600) to enhance steric stability, SDS to enhance electrostatic stability, and sodium alginate to enhance both the steric and electrostatic stabilities. The colloids were characterized by UV-vis spectrophotometry and transmission electron microscopy. The average diameters of 1.8AuNPs and 4.0AuNPs were 13.33 and 18.86 nm, corresponding to the absorbance peak at 533 and 539 nm, respectively. The colloidal stability was studied using the results from spectrophotometry. The aggregation of as-synthesized AuNPs, reflected by the red shift of the spectrum, occurred after the centrifugation at 10000 rpm for 10 minutes, but this was prevented after coating with 5%w/v PEG400, 5%w/v PEG6000, 0.1%w/v alginate, and 0.5%w/v SDS solutions. Electrostatic stabilization using alginate and SDS could withstand up to three times of centrifugation, while steric stabilization using PEG400 and PEG6000 could withstand the disturbance of Ca2+ more effectively. The smaller AuNPs were more affected by Ca2+ than the larger ones. In highly acidic solutions, all coated AuNPs showed the red shift and SDS-coated AuNPs were the most effective against acid disturbance, and in highly basic solutions, all coated AuNPs showed the blue shift. Besides, the coated AuNPs were still stable at 52ºC. Furthermore, the theoretical study of growth and nucleation of AuNPs was done. The modeling of nucleation and growth via a coalescence of two nanoparticles were simplified using reaction kinetics of the diffusion-controlled reaction. The frequency factor and activation energy were approximated and the set of differential equations were solved using Polymath. The concentrations of nanoparticles of discretized radii ranging from 1 to 12 nm were obtained at different reaction times. Therefore, the size distribution of the AuNPs could be obtained. Qualitatively, the effect of the concentration of gold precursor (1.8 and 4.0 mM) on the size distributions was observed consistent with the experimental results.

本研究以冷萃废弃咖啡渣提取物为原料合成金纳米颗粒（Gold nanoparticles，AuNPs）。本研究分别采用1.8 mM氯金酸（HAuCl4）与4.0 mM氯金酸制备了两种平均粒径的AuNPs（分别记为1.8AuNPs与4.0AuNPs），并对比二者在机械、化学、热学及酸碱扰动下的胶体稳定性提升效果。此外，本研究通过包覆改性进一步优化AuNPs的胶体稳定性：以聚乙二醇400与6000（polyethylene glycol 400 and 6000，PEG400与PEG6000）提升空间位阻稳定性，以十二烷基硫酸钠（Sodium dodecyl sulfate，SDS）提升静电稳定性，以海藻酸钠同时实现空间位阻与静电双重稳定。采用紫外-可见分光光度法与透射电子显微镜对所得胶体进行表征：1.8AuNPs与4.0AuNPs的平均粒径分别为13.33 nm与18.86 nm，对应紫外吸收峰分别位于533 nm与539 nm处。本研究通过分光光度法表征结果评估胶体稳定性：未包覆的原始AuNPs经10000 rpm离心10分钟后会发生团聚，该现象可通过光谱红移体现；但经5%w/v PEG400、5%w/v PEG6000、0.1%w/v海藻酸钠及0.5%w/v SDS溶液包覆后，团聚现象可被有效抑制。采用海藻酸钠与SDS构建的静电稳定体系可耐受至多3次离心处理，而PEG400与PEG6000构建的空间位阻稳定体系对钙离子（Ca²+）扰动的抵抗能力更强；相较于大粒径AuNPs，小粒径AuNPs受钙离子扰动的影响更为显著。在强酸性溶液中，所有包覆型AuNPs均出现光谱红移，其中SDS包覆的AuNPs抗酸扰动效果最优；而在强碱性溶液中，所有包覆型AuNPs均出现光谱蓝移。此外，包覆型AuNPs在52℃条件下仍可保持稳定。本研究同时开展了AuNPs成核与生长过程的理论模拟：通过扩散控制反应动力学简化了基于纳米颗粒团聚的成核与生长模型，对频率因子与活化能进行了近似估算，并通过Polymath软件求解了微分方程组，获取了不同反应时刻下粒径离散化分布在1~12 nm范围内的AuNPs浓度数据，由此可得到AuNPs的粒径分布情况。定性分析表明，金前驱体浓度（1.8 mM与4.0 mM）对粒径分布的影响趋势与实验结果一致。