Real-Time Probing of Melamine-Induced Gold Nanoparticle Aggregation Kinetics via Evanescent-Wave Coupled Cavity Ring-down Spectroscopy

Abstract

Insight into the aggregation kinetics of gold nanoparticles (GNPs) is critical for developing a colorimetric assay extensively used in chemical and biomolecular sensing. The aggregation of NPs plays a significant role in many natural and industrial processes, demanding comprehensive perceptions of the aggregation kinetics at a solid–liquid interface. However, the direct observation of the melamine-induced aggregation process of GNPs in the time-domain still remains a challenge. There is little to no information on the fundamental mechanisms of such kinetics using evanescent waves. Total internal reflection (TIR) has been applied to generate the evanescent field (EF), exploring aggregation kinetics near the solid–liquid interface. Here, we utilized a precise optical cavity-based method, an evanescent-wave coupled cavity ring-down spectroscopy (EW-CRDS), that can probe the melamine-induced aggregation kinetics of GNPs. The key feature of the present method is that the evanescent field generated by TIR illumination harnesses the power of CRDS to study 2D fractals via the collision and attachment of the GNPs and their melamine-induced aggregates at the interfacial region in real-time. This kinetic study reveals a critical point for diffusion-limited aggregation and provides insights into the design and optimization of colorimetric sensors that exploit the aggregation of GNPs. Furthermore, the EW-CRDS is a unique analytical approach that helps to deepen our understanding in probing the real-time aggregation process, detecting the presence of aggregator as compared to UV–vis and dynamic light scattering (DLS) spectroscopy.

Anomalous scattering of polystyrene microparticles revealed by evanescent wave coupled cavity ringdown spectroscopy

“…The evanescent wave is non-propagating, non-radiative, and decaying, as defined by the e − kz term instead of e − ikz for propagating electromagnetic waves. 17 The penetration depth ( d p ) of the evanescent wave in the rarer medium (in the present setup, the rarer medium is water at the glass–water interface) is given by 27 where λ 0 is the incidence wavelength, n p and n s are the refractive index of the prism (1.515@638 nm) and water (1.330 for water), respectively (critical angle, θ c = 61.38°). The penetration depth of the EW for the present configuration ( θ ∼ 72.84°) was d p ∼ 89 nm, calculated using eqn (8).…”

“…The noise-equivalent absorption (NEA) coefficient is , where the data sampling rate ( f s ) is 100 MHz, σ τ is the mean standard deviation in the fit-determined RDT, τ is the measured RDT, and c is the velocity of light in free space. The ringdown times are converted to the absorbance ( A ) or extinction values using the following equation: 17 where τ 0 and τ are the RDTs (in ns) for the baseline solution and the sample solution, respectively. Here, the effective path length ( L eff ) is the combination of the light pulse path length in the air inside the cavity and the path length inside the prism ( L eff = L air n air + L prism n prism ).…”

“…15,16 The evanescent wave-coupled CRDS (EW-CRDS) technique is still evolving and provides an opportunity for significant development, both for improved selectivity and extended sensitivity. 17–21 The penetration depth of the evanescent wave is of the order of ∼100 nm, accounting for the water phase of biologically relevant samples. The EW-CRDS technique based on the total internal reflection (TIR) configuration provides sufficient accuracy to detect correlation effects even for strongly turbid samples like aqueous PSM solutions.…”