Ultrasmall gold nanoparticles were synthesized without strong capping agents by using a capillary-based continuous flow system. A mixture of gold(III) chloride trihydrate and trisodium citrate flowed through capillaries at elevated temperature. The effect of capillary material (polytetrafluoroethylene, fluorinated ethylene propylene, polyetheretherketone, fused silica), surface-to-volume ratio (capillary internal diameter 0.3–1 mm), average residence time (1.5–30 min) and temperature (70–100 °C) were investigated. At a flow rate of 0.006 ml/min (residence time 30 min), 100 °C, 275 kPa back pressure, citrate/gold molar ratio 3.15 and using PTFE capillary tubing with an inner diameter of 0.3 mm, very small (1.9 ± 0.2 nm) nanoparticles were obtained. For comparison, experiments were also performed under the same experimental conditions, but in slug flow using octane as segmenting fluid, thus isolating the reactants from the tubing wall. The synthesized particles were 17.4 ± 1.4 nm for segmented flow, demonstrating the important effect of the capillary wall surface. The performance of these citrate-capped gold nanoparticles was tested for Surface-Enhanced Raman Scattering (SERS). The average enhancement factor (AEF) of 2 nm gold nanoparticles capped by citrate from our work (AEF = 1.54 × 108) was nearly double when compared to 2 nm phosphate-capped commercial gold nanoparticles (AEF = 7.34 × 107). The adsorption of analyte molecules onto citrate-capped gold surface was easier due to the weaker binding strength of the carboxylate ligand and more hotspots formed with narrower gaps between neighbouring particles, giving rise to improved enhancement. This work has been selected by the Editors as a Featured Cover Article for this issue.

中文翻译：

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使用柠檬酸三钠在微反应器中连续流动合成超小金纳米颗粒及其 SERS 性能

通过使用基于毛细管的连续流动系统，在没有强封端剂的情况下合成了超小金纳米粒子。氯化金 (III) 三水合物和柠檬酸三钠的混合物在升高的温度下流过毛细管。毛细管材料（聚四氟乙烯、氟化乙烯丙烯、聚醚醚酮、熔融石英）、表面体积比（毛细管内径 0.3-1 毫米）、平均停留时间（1.5-30 分钟）和温度（70-100°）的影响C) 进行了调查。流速为 0.006 ml/min（停留时间 30 min）、100 °C、275 kPa 背压、柠檬酸盐/金摩尔比为 3.15 并使用内径为 0.3 mm、非常小的 PTFE 毛细管（1.9 ± 0.2 nm) 纳米颗粒。为了比较，在相同的实验条件下也进行了实验，但在段塞流中使用辛烷作为分段流体，从而将反应物与管壁隔离。合成的颗粒为 17.4 ± 1.4 nm，用于分段流动，表明毛细管壁表面的重要作用。对这些柠檬酸盐封端的金纳米粒子的性能进行了表面增强拉曼散射 (SERS) 测试。与 2 nm 磷酸盐封端的商业金纳米粒子（AEF = 7.34 × 107）相比，我们工作中柠檬酸盐封端的 2 nm 金纳米粒子的平均增强因子（AEF）（AEF = 1.54 × 108）几乎是两倍。由于羧酸盐配体的结合强度较弱，并且由于相邻颗粒之间的间隙较窄而形成更多热点，因此更容易将分析物分子吸附到柠檬酸盐封端的金表面，从而提高增强效果。