Department of Mechanical Engineering, University of California Santa Barbara , Santa Barbara, California 93106, United States.
Langmuir. 2013 Nov 5;29(44):13614-23. doi: 10.1021/la400949x. Epub 2013 Oct 22.
The aggregation kinetics of silver nanoparticles in sessile droplets were investigated both experimentally and through numerical simulations as a function of temperature gradient and evaporation rate, in order to determine the hydrodynamic and aggregation parameters that lead to optimal surface-enhanced Raman spectroscopic (SERS) detection. Thermal gradients promote effective stirring within the droplet. The aggregation reaction ceases when the solvent evaporates forming a circular stain consisting of a high concentration of silver nanoparticle aggregates, which can be interrogated by SERS leading to analyte detection and identification. We introduce the aggregation parameter, Γa ≡ τ(evap)/τ(a), which is the ratio of the evaporation to the aggregation time scales. For a well-stirred droplet, the optimal condition for SERS detection was found to be Γ(a,opt) = kc(NP)τ(evap) ≈ 0.3, which is a product of the dimerization rate constant (k), the concentration of nanoparticles (cNP), and the droplet evaporation time (τ(evap)). Near maximal signal (over 50% of maximum value) is observed over a wide range of aggregation parameters 0.05 < Γa < 1.25, which also defines the time window during which trace analytes can be easily measured. The results of the simulation were in very good agreement with experimentally acquired SERS spectra using gas-phase 1,4-benzenedithiol as a model analyte.
研究了固支液滴中银纳米粒子的聚集动力学，实验和数值模拟均研究了温度梯度和蒸发率的影响，以确定导致最佳表面增强拉曼光谱（SERS）检测的流体力学和聚集参数。热梯度促进了液滴内的有效搅拌。当溶剂蒸发形成由高浓度银纳米粒子聚集体组成的圆形斑点时，聚集反应停止，SERS 可对其进行检测，从而实现对分析物的检测和识别。我们引入了聚集参数Γa≡τ(evap)/τ(a)，即蒸发和聚集时间尺度的比值。对于搅拌良好的液滴，发现 SERS 检测的最佳条件为Γ(a,opt)=kc(NP)τ(evap)≈0.3，这是二聚化速率常数（k）、纳米粒子浓度（cNP）和液滴蒸发时间（τ(evap)）的乘积。在聚集参数 0.05<Γa<1.25 的较宽范围内，观察到接近最大信号（超过最大值的 50%），这也定义了可以轻松测量痕量分析物的时间窗口。模拟结果与使用气相 1,4-苯二硫醇作为模型分析物获得的实验 SERS 光谱非常吻合。
