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.
This work exploits the localized surface plasmon resonance (LSPR) spectroscopy of noble metal nanoparticles to achieve sensitive and selective detection of biological analytes. Noble metal nanoparticles exhibit an LSPR that is strongly dependent on their size, shape, material, and the local dielectric environment. The LSPR is also responsible for the intense signals observed in surface-enhanced Raman scattering (SERS). Ag nanoparticles fabricated using the nanosphere lithography (NSL) technique exploits this LSPR sensitivity as a signal transduction method in biosensing applications. The current work implements LSPR biosensing for the anti dinitrophenyl (antiDNP) immunoassay system. Upon forming the 2,4 dinitrobenzoic acid/antiDNP complex, this system shows a large LSPR shift of 44 nm when exposed to antiDNP concentration of 1.5 x 10<sup>-6</sup> M. In addition, due to the unique molecular characteristics of the functional groups on the biosensor, it can also be characterized using SERS. First, the nanoparticles are functionalized with a mixed self-assembled monolayer (SAM) comprised of 2:1 octanethiol and 11-amino undecanethiol. The SAM is exposed to 2,4-dinitrobenzoic acid with the 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC) coupling reagent. Finally, the 2,4-dinitrophenyl terminated SAM is exposed to various concentration of antiDNP. LSPR shifts indicate the occurrence of a binding event. SER spectra confirm binding of 2,4 dinitrobenzoic acid with amine-terminated SAM. This LSPR/SERS biosensing method can be generalized to a myriad of biologically relevant systems.
Metal film over nanosphere (MFON) electrodes are excellent substrates for surface-enhanced Raman scattering (SERS) spectroscopy. These surfaces are produced by vapor deposition of a metal film over nanospheres that are assembled in a hexagonally close packed arrangement. The efficiency and reproducibility of AgFON electrode as SERS substrates are confirmed by the repeatability of the electrochemical surface enhanced Raman scattering spectra of pyridine and the Ru(bpy)<sub>3</sub><sup>3+</sup>/Ru(bpy)<sub>3</sub><sup>2+</sup> complexes adsorbed on AgFON electrodes. The Raman signal for AgFON electrodes is observed to be extremely stable even at extremely negative potentials in both aqueous and nonaqueous electrolytes. Recent reports have indicated that SERS enhancement factors of up to 14 orders of magnitude can be achieved, providing the sensitivity requisite for ultra trace level detection of target analytes. For this reason, we are developing a method for bacterial endospore SERS detection based on the endospores marker -- dipicolinic acid (DPA). The SERS spectra of dipicolinic acid in aqueous solutions are reported. The dipicolinate vibrational features could be observed in the SERS spectra at the concentration as low as 8 × 10<sup>-5</sup> M in 5 minutes. These limits of detection are entirely controlled by the thermodynamics and kinetics of DPA binding to the AgFON surface.