Directed Assembly of Au Nanostar@Ag Satellite Nanostructures for SERS-Based Sensing of Hg2+ Ions

Embedding Raman reporters within nanosized gaps of metallic nanoparticles is an attractive route for surface-enhanced Raman spectroscopy (SERS) applications, although often this involves complex synthesis procedures that limit their practical use. Herein, we present the tip-selective direct growth of silver satellites surrounding gold nanostars (AuNSt@AgSAT), mediated by a dithiol Raman reporter 1,4-benzenedithiol (BDT). We propose that BDT is embedded within nanogaps which form between the AuNSt tips and the satellites, and plays a key role in mediating the satellite growth. Not only proposing a rationale for the mechanistic growth of the AuNSt@AgSAT, we also demonstrate an example for its use for the detection of Hg2+ ions in water. The presence of Hg2+ resulted in amalgamation of the AuNSt@AgSAT, which altered both its structural morphology and Raman enhancement properties. This provides a basis for the detection where the Raman intensity of BDT is inversely proportional to the Hg2+ concentrations. As a result, Hg2+ could be detected at concentrations as low as 0.1 ppb. This paper not only provides important mechanistic insight into the tip-selective direct growth of the anisotropic nanostructure but also proposes its excellent Raman enhancement capability for bioimaging as well as biological and chemical sensing applications.


Finite Element Method (FEM) based computational modelling:
Solutions were generated by solving Maxwell's wave equations in a frequency-domain study. The incident wavelength used was 785 nm (similar to the excitation laser wavelength used in SERS experiments). Incident electric field can be written as: To solve for electric field, Maxwell's electromagnetic wave equation can be expressed as: Where 0 is the wavenumber in free space, μr is the relative permeability of the medium, is the permittivity of the medium, j is current density, Esc is the scattered field, and is the electrical conductivity of the medium. Now using the Mie theory, the incident energy flux was calculated as: Model Building: To represent the AuNSt and AuNSt@AgSAT nanoparticles, COMSOL Multiphysics modelling was performed. The TEM image of AuNSt@AgSAT was replicated for making the structures, and all the parameters were calculated using Image J software (see table S1. Core-shell was built, considering it an ellipse of the semi-major and minor axis as 39.00 ± 1.21 and 33.50 ± 1.35 nm, respectively. Tips were made by adding two different structures; ellipse as tip head situated over polygon structure as a base. AgSATs were built at the tip heads, giving a gap of 1.58 nm (as measured in TEM images). Optical properties of Au and Ag metals were acquired from Johnson et al. 1 and Rakic A et al. 2 respectively. Scattering boundary conditions were implied at both left and right sides of the simulation region, and perfectly matched layers (PMLs) of 300 nm were added at the top and bottom in order to absorb any unwanted radiation. All the domains were meshed using a custom setting. The star shape and the AgSAT parts were meshed with a maximum element size of 2 nm and maximum element growth size of 1.7. Since water is the surrounding medium the refractive index for the surrounding was kept as 1.33. The schematic diagram and mesh distribution can be seen in figure S1 and figure S2.

Calculation of simulated EF
The total enhancement is a sum of (i) Electromagnetic enhancement (ii) Chemical enhancement. The COMSOL Multiphysics simulates the electromagnetic enhancements part of the SERS effect. The electromagnetic enhancement is a two-step process. The first enhancement takes place with the interaction of ( 0 ) with the nanostructure. EF of the first step is: The second step of enhancement occurs when the localized field at the Raman frequency from the first step interacts with the nanostructure once again. EF of the second step at the Raman frequency is: For low-frequency vibrational of adsorbed molecules, the ( 0 ) ≈ ( ), so G1( 0 )≈ G2( ).
So the total electromagnetic EF, Calculation of maximum Enhancement factor: (1) For AuNSt; EF of the first step at the incident frequency 0 ; = .
EF of the second step at the Raman frequency ; = .
So, the total electromagnetic EF from AuNSt; = .
EF of the second step at the Raman frequency ; @ = .