The extinction maximum of the localized surface plasmon resonance (LSPR) of noble metal nanoparticles is highly
dependent upon the refractive index of the nanoparticles' surrounding environment. In this study, the effect that
molecular resonances have on the intensity, LSPR peak width, and LSPR shift of the LSPR of Ag nanoparticles is
monitored. By systematically tuning the LSPR extinction maxima of Ag nanoparticles versus molecular resonances,
new phenomena are revealed. First, the LSPR peak shift induced by a resonant molecule varies with wavelength. In
most instances, the trends in this data qualitatively track with the Kramer's-Kronig transformation of the molecular
resonance spectrum; however, the magnitude of the response is severely underestimated. This was verified from both
experimental data and theoretical calculations. Because this phenomenon is revealed to be electronic transition
dependent, it is hypothesized that the coupling between the molecular and plasmon resonances is responsible for this
wavelength dependent observation. These results will have implications in molecular enhanced LSPR sensing and in the
understanding of surface-enhanced spectroscopy.
Localized surface plasmon resonance (LSPR) is one of the signature optical properties of noble metal nanoparticles. Since the LSPR wavelength λ<sub>max</sub> is extremely sensitive to the local environment, it allows us to develop nanoparticle-based LSPR chemical and biological sensors. In this work, we tuned the LSPR peaks of Ag nanotriangles and explored the wavelength-dependent LSPR shift upon the adsorption of some resonant molecules. The induced LSPR peak shifts (Δλ<sub>max</sub>) vary with wavelength and the line shape of the LSPR shift is closely related to the absorption features of the resonant molecules. When the LSPR of the nanoparticles directly overlaps with the molecular resonance, a very small LSPR shift was observed. An amplified LSPR shift is found when LSPR of the nanoparticles is at a slightly longer wavelength than the molecular resonance of the adsorbates. Furthermore, we apply the "amplified" LSPR shift to detect the substrate binding of camphor to the heme-containing cytochrome P450cam protiens (CYP101). CYP101 absorb light in the visible region. When a small substrate molecule binds to CYP101, the spin state of the molecule is converted to its low spin state. By fabricating nanoparticles with the LSPR close to the molecular resonance of CYP101 proteins, the LSPR response as large as ~60 nm caused by the binding of small substrate has been demonstrated.
The intense color of noble metal nanoparticles has inspired artists and fascinated scientists for hundreds of years. These rich hues are due to the interaction of light with the nanostructure's localized surface plasmon (LSPR). Here, we describe three optical sensing modalities that are dependant on the effects of the LSPR. Specifically, we will demonstrate the use of LSPR supporting particles as analogues to fluorescent probes and labels for multiplex detection, sensing based on observation of changes in the LSPR spectrum caused by alteration of the local refractive index upon analyte binding, and the spectroscopic labeling of cells and tissues with Surface Enhanced Raman Scatting (SERS) active nanoparticles probes.
Recently, nanoparticles have become the platform for many sensing schemes. In particular, the utilization of the optical response of nanoparticles to changes in their nanoenvironment has served as a signal transduction mechanism for these sensing events. For example, silver nanoparticle arrays synthesized using nanosphere lithography have served as an ultrasensitive detection platform for small molecules, proteins, and antibodies with the detection limit of 60,000 and less than 25 molecules/nanoparticle for hexadecanethiol and antibodies, respectively. While this approach is low cost and highly portable, one limitation of the array platform is that the signal arises from approximately 1x10<sup>6</sup> nanoparticles. A method to improve the overall number of molecules detected would be to decrease the number of nanoparticles probed. Recently, single nanoparticle sensing has been accomplished using dark-field microscopy. A 40 nm shift in the localized surface plasmon resonance induced from less than 60,000 small-molecule adsorbates has been monitored from a single Ag nanoparticle. Additionally, streptavidin sensing has also been demonstrated using a single Ag nanoparticle. Detection platforms based on nanoparticle arrays and single nanoparticles will be discussed and compared.
The Ag nanoparticle based localized surface plasmon resonance (LSPR) nanosensor yields ultrasensitive biodetection with extremely simple, small, light, robust, and low-cost instrumentation. Using LSPR spectroscopy, the model system, biotinylated surface-confined Ag nanotriangles, was used to detect less than one picomolar up to
micromolar concentrations of streptavidin. Additionally, the monitoring of anti-biotin binding to biotinylated Ag nanotriangles exhibited that the system could be used as a solution immunoassay. The system was rigorously tested for nonspecific binding interactions and was found to display virtually no adverse results. These results represent important new steps in the development of the LSPR nanobiosensor for applications in medical diagnostics, biomedical research, and environmental science.