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Silver nanoparticles (AgNPs) coated with stimuli-responsive polymers are promising hybrid nanomaterials for plasmonic and sensing applications [1]. Polymer shells provide steric stabilization and allow the nanoparticle surface to be functionalized [2]. In this work, AgNPs coated with poly(N-isopropylacrylamide) (pNIPAM) were synthesized by a photochemically assisted method. The order in which reagents were added and the UV irradiation time were varied to optimize nanoparticle formation and shell growth.
Atomic force microscopy confirmed the formation of well-defined nanoparticles. Dynamic light scattering gave hydrodynamic size distributions consistent with the presence of a polymer coating. Light extinction spectra showed the localized surface plasmon resonance band characteristic of AgNPs.
To interpret the spectra quantitatively, we developed a computer model that estimates distributions of core size and polymer shell thickness from extinction measurements. The modeling had two stages:
1. Spectrum generation. Extinction spectra were calculated for bare and pNIPAM-coated AgNPs and for pure pNIPAM particles, covering a range of core sizes and shell thicknesses. The calculations used Mie theory for coated spheres [3] with the optical constants of bulk Ag were approximated by high-order polynomials and corrected for size-dependent electron relaxation, using the bulk relaxation time, plasma frequency, and Fermi velocity of silver. The refractive index of pNIPAM was taken as 1.52.
2. Spectrum fitting. The simulated spectra were combined with weighting coefficients chosen by Monte Carlo enumeration. The summed spectrum was then fitted to the experimental data by a least-squares technique.
The simulated spectra agreed well with the experimental ones and allowed the structural parameters of the nanoparticles to be estimated. The proposed photochemical method yields stable colloids with a controlled size distribution. These nanoparticles are promising in fabricating nanocontainers for drug delivery and as signal enhancers in plasmon-enhanced spectroscopy.
References:
1. Diehl, F., Hageneder, S., Fossati, S., Auer, S. K., Dostalek, J., & Jonas, U. (2022). Plasmonic nanomaterials with responsive polymer hydrogels for sensing and actuation. Chemical Society Reviews, 51(10), 3926-3963.
2. Duan, H., Muhuri, D., & He, J. (2024). Polymers on plasmonic metal nanoparticles: From symmetric coating to asymmetric surface patterning. Polymer, 303, 127115.
3. Lopatynskyi, A. M., Lopatynska, O. G., Guo, L. J., & Chegel, V. I. (2010). Localized surface plasmon resonance biosensor—Part I: Theoretical study of sensitivity—Extended Mie approach. IEEE Sensors Journal, 11(2), 361-369.
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