Enhancing Label-Free Biosensing With Cryogenic Temperature-Induced Plasmonic Structures

Plasmonic nanoparticles exhibit distinct nearfield properties that offer potential advantages for label-free biosensing applications. By generating strong interactions between light and matter, these nanoparticles can be employed to detect the presence of analytes through monitoring spectral variations within the plasmonic resonances resulting from changes in the biomolecules on their surface. Various fabrication methods have been introduced in the literature to produce such particles. In this study, precise control over the shape, size, and distribution of nanoparticles is crucial to effectively excite plasmonic resonances with desirable optical properties. Traditionally, the vacuum deposition technique has been widely used to create nanoparticles with good crystalline properties and uniform particle distribution at high substrate temperatures (300 K). However, this method exhibits reduced surface or volume diffusion of particles at lower substrate temperatures, leading to a decrease in the particle size of the metal thin films. Additionally, surface homogeneity deteriorates due to the shadowing effect when dealing with thicker film coatings. To overcome these limitations, we propose a cryogenic temperature method, in which vacuum evaporation is performed at low substrate temperatures (200 K). Our method demonstrates the ability to produce nanoparticle systems with homogeneous particle size distribution and high structural quality. To evaluate the efficacy of our approach for biosensing applications, we compared two silver (Ag) nanoparticle systems prepared using the classical vacuum deposition technique (300 K) and our cryogenic temperature method (200 K) in terms of their structural and optical properties. We showed that our method enables the excitation of plasmonic resonances with narrower linewidths compared to the classical evaporation technique. Moreover, our method allows for the realization of nanoparticles with more controlled dimensions, which are homogeneously distributed over the substrate surface. This characteristic facilitates the generation of surface plasmon excitations associated with large local electromagnetic fields that extend extensively into the volume surrounding the sensing surface. Therefore, our method facilitates stronger light-matter interactions compared to the classical technique, resulting in enhanced refractive index sensitivity for label-free biosensing applications. We conducted sensing experiments using bulk solutions with different refractive indices to demonstrate the superior refractive index sensitivity of our cryogenic temperature method. Our findings indicate that our method produces plasmonic nanoparticle systems with higher refractive index sensitivities. Furthermore, when functionalizing the metal surface with protein mono- and bilayers, our method yields larger spectral shifts within the plasmonic resonances compared to the classical vacuum deposition technique. This indicates that our method has the potential to serve as a highly sensitive and label-free plasmonic biosensing platform without the need for expensive, slow, and complex fabrication techniques that require a sophisticated clean-room infrastructure. In summary, we believe that our method opens up possibilities for the development of advanced plasmonic biosensing platforms with exceptional sensitivity.