Understanding multiscale assembly mechanism in evaporative droplet of gold nanorods

Abstract

Using in-situ contact angle measurement and ex-situ scanning electron microscopy, we present a direct correlation of the drying profile of CTAB coated AuNRs suspension with observed deposition pattern on hydrophilic silicon substrate. The droplet evaporation reveals the formation of surface pattern that primarily exhibits three different regions of assembly. Region 1, the exterior of droplet's triple phase contact line (TPCL), consists of small arrays of AuNR cluster followed by the much observed multi-layered coffee-ring deposit, comprising of AuNRs assembled in smectic phase. The transient TPCL depinning during the initial stage of evaporation due to autophobic effect results in formation of region 1. The increase in AuNR concentration due to outward capillary flow leads to pinned TPCL and coffee-ring deposit in region 2. The depletion mediated interparticle interaction due to nanoparticle stagnation at TPCL results in surfactant expulsion. The expelled CTAB creates surfactant-induced Marangoni flow and results in the formation of depletion region 3. Further, we will discuss the plasmonic application of the deposit pattern in determination of surface-enhanced Raman scattering signal. The SERS signal enhancement of Rhodamine B analyte, by factor of ~7.6 ∗ 105 and detection limit to concentration as low as ~10−8M has been achieved using the ordered arrays of gold nanorods as substrates. The AuNR assembly can be further explored for fabrication of optical sensors and other application based studies.