While the optical and spectroscopic properties of gold nanoparticles are widely used for chemical, bioanalytical, and biomedical applications, the study of the size correlation with these properties for nanoparticles in solutions is rather limited. This paper describes the results of a systematic study of such a correlation for gold nanoparticles with diameters ranging from 10 to 100 nm in aqueous solutions. The high monodispersity of these nanoparticles permitted a meaningful correlation of the particle size with the surface plasmon (SP) resonance band properties and the surface-enhanced Raman scattering (SERS) spectroscopic properties. This correlation is compared to the results from the simulation based on Mie theory. The close agreement between the experimental and the theoretical results provides insight into the validity of determining the wavelength of the SP resonance band as a measure of the particle size. The size correlation with the SERS intensity from the adsorption of 4-mercaptobenzoic acid on the nanoparticles in aqueous solutions reveals the existence of a critical size of the nanoparticles in the solution beyond which the particle−particle interaction is operative and responsible for the SERS effect. These findings serve as the basis of size correlations for exploiting the optical and spectroscopic properties of gold nanoparticles of different sizes in aqueous solutions in analytical or bioanalytical applications.
The ability to create bio-functional nanoprobes for the detection of biological reactivity is important for developing bioassay and diagnostic methods. This paper describes the findings of an investigation of the surface functionalization of gold (Au) and magnetic nanoparticles coated with gold shells (M/Au) by proteins and spectroscopic labels for the creation of nanoprobes for use in surface enhanced Raman scattering (SERS) assays. Highly monodispersed Au nanoparticles and M/Au nanoparticles with two types of magnetic nanoparticle cores (Fe(2)O(3) and MnZn ferrite) were studied as model systems for the bio-functionalization and Raman labeling. Comparison of the SERS intensities obtained with different particle sizes (30-100 nm) and samples in solution versus on solid substrates have revealed important information about the manipulation of the SERS signals. In contrast to the salt-induced uncontrollable and irreversible aggregation of nanoparticles, the ability to use a centrifugation method to control the formation of stable small clustering sizes of nanoparticles was shown to enhance SERS intensities for samples in solution as compared with samples on solid substrates. A simple method for labeling protein-capped Au nanoparticles with Raman-active molecules was also described. The functionalized Au and M/Au nanoparticles are shown to exhibit the desired functional properties for the detection of SERS signals in the magnetically separated reaction products. These results are discussed in terms of the interparticle distance dependence of 'hot-spot' SERS sites and the delineation of the parameters for controlling the core-shell reactivity of the magnetic functional nanocomposite materials in bio-separation and spectroscopic probing.
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