Hottest Hotspots from the Coldest Cold: Welcome to Nano 4.0
Aayush Rai, Seemesh Bhaskar, Kalathur Mohan Ganesh, Sai Sathish Ramamurthy
Abstract
Conventionally, nanoassemblies are synthesized using widely adopted template-based approaches. External stimuli such as electric and magnetic fields and light-induced reactions have recently been investigated. The exploration using a temperature gradient in this spotlight is very nascent. In this context, soret colloids are nanoparticle (NP) assemblies obtained via adiabatic cooling at −18 °C. They have found widespread utility in surface plasmon-coupled emission (SPCE) and surface-enhanced Raman scattering (SERS) for sensing analytes and ions of biological and environmental concern. However, the drawback of the current methodology for obtaining enhanced tunability in functional properties for large-scale production has remained a bottleneck hitherto on account of the significant time (2 h) needed for building precise nanoassemblies. In this direction, a rapid, one-pot, and cost-effective adiabatic cooling methodology to obtain precise nanoassemblies with exceptionally tunable optical and morphological properties is experimentally demonstrated in this work. Thermodiffusion of homogeneous gold (Au) and silver (Ag) nanoparticles using adiabatic cooling at cryoshift temperatures (−80, −150, and −196 °C (or liquid N2)) significantly lowered the time from 2 h to 3 min for obtaining structurally and functionally tunable nanoassemblies. This methodology aids in the realization of hotspots of first, second, third, and fourth generations, which are nanoregimes of high electric-field intensity. The innovative fourth-generation hotspots (a horizon toward Nano 4.0) were distinctively generated and studied by mounting the cryosorets on SPCE substrates. The dual dependence of nanoassembly formation on time and cooling temperature is elaborately discussed for the first time in this study. The abundant field intensity and synergistic plasmon hybridization between metallic Ag, dielectric TiO2 nanorods, and graphene oxide π-plasmons assisted in the realization of single-molecule detection. The sensing is achieved using a cost-effective smartphone-based technology that is amenable to resource-limited settings. This work opens a window to accomplish precise nanoassemblies of different sizes, shapes, material properties, numbers of particles by modulating the adiabatic cooling time and temperature, for use in biosensing, photonics, and interdisciplinary applications.