Self-Assembly Gold Nanoislands for Localized Surface Plasmon Resonance Sensor Applications

Abstract

Localized surface plasmon resonance (LSPR) is a strong optically exited electromagnetic near-field effect associated with noble metal nanostructures. Other than the propagating electromagnetic wave of surface plasmon polaritons (SPP), LSPR nanostructures confine the plasmonic oscillation into the range comparable to the wavelength of light and greatly enhance the electric fields near the nanoparticles. Owing to the locally enhanced electromagnetic field confined to the vicinity of the nanostructures at resonance, array of plasmonic nanostructures could demonstrate high sensitivity to local refractive index (RI) variation. Thus, LSPR is an ideal candidate for real-time label-free sensing applications of analytical chemistry and biomolecule detection.

Self-assembly gold nanoislands (SAM-AuNIs), which form a randomly distributed nanostructure, are fabricated by a two-step thin-film deposition-annealing method. Despite random distribution of the SAM, the p-polarized light after total internal reflection shows significant phase transition at the extinction wavelengths upon refractive index variation due to LSPR effect. It exhibits the strong plasmonic transition than that observed in conventional surface plasmon resonance (SPR) sensors. In the beginning of my thesis, I presented the experimental investigations to the SAM-AuNIs LSPR sensor and their excellent performance on label-free sensing applications.

In case of the LSPR biosensing application, functionalized nano-gold has been the most popular platform for biosensing such as immune antibody/antigen interaction and DNA hybridization. The biomolecular receptors are immobilized onto the surface of gold nanostructure with activated 11-mercaptoundecanoic acid, so that the localized refractive index change induced by binding event can be registered as alteration of the resonance frequency with the transducing light waves. Currently, the extensive experimental and computational studies suggested that the most intensified electric fields known as “hot spots” are due to the coupled resonance effect and these “hot spots” are located in the dielectric gaps of the plasmonic nanoparticles. These “hot spots” are believed to be highly sensitive to localized refractive index change or binding events. My work on dielectric functionalization approach had attained excellent sensing performance of detecting human IgG antigen in both PBS buffer and human serum. In view of the myriad of emerging plasmonic materials in the foreseeable future, the dielectric functionalization approach would provide a convenient protocol for label-free LSPR biosensing, as it would be generally applicable to any plasmonic material on glass substrate.

Here I also report a novel label-free detection scheme with nickel-doped graphene (NDG) as the functionalized receptor on the SAM-AuNIs sensors. The NDG functionalized AuNIs biosensor exhibited good sensitivity and selectivity toward 3-nitro-L-tyrsine (3-NT), a biomarker of neurodegredative diseases. When compared with reported 3-NT immunoassay with enzyme-linked immunosorbent assay (ELISA), the developed NDG-AuNIs platform offers two advantages i.e. 1) label-free and 2) direct chemisorption of 3-NT. In addition, the adsorption of 3-NT to the NDG receptor were also investigated by atomic force microscopy and further verified by surface enhanced Raman spectroscopy. Therefore, the NDG-LSPR biosensor competes favorably against ELISA and it is believed to be an attractive and economical solution to early diagnostic of 3-NT related disorders for clinical applications.

Using the established SAM-AuNIs functionalized by poly(m-phenylenediamine-co-aniline-2-sulfonic acid) (poly(mPD-co-ASA)) copolymer nanoparticles as specific receptors, a highly sensitive LSPR optochemical sensor was demonstrated for detection of trace lead cation (Pb(II)) in drinking water. The copolymer receptor was optimized in three aspects: (1) mole ratio of mPD:ASA monomers, (2) size of copolymer nanoparticles, and (3) surface density of the copolymer. It was shown that the 95:5 (mPD:ASA mole ratio) copolymer with size less than 100 nm exhibited the best Pb(II)-sensing performance, and the 200 times diluted standard copolymer solution contributed to the most effective functionalization protocol. The resulting poly(mPD-co-ASA)-functionalized LSPR sensor attained detection limit to 0.011 ppb toward Pb(II) in drinking water, and the linear dynamic range covers 0.011 to 5000 ppb (i.e., 6 orders of magnitude). In addition, the sensing system exhibited robust selectivity to Pb(II) in the presence of other metallic cations as well as common anions. The proposed functional copolymer functionalized on AuNIs was found to provide excellent Pb(II)-sensing performance using simple LSPR instrumentation for rapid drinking-water inspection. This work had also led to a patent application.

The SAM-LSPR chip consumes only one-tenth of gold when compared with that used in thinfilm SPR sensor. Yet, the developed SAM-AuNIs was successfully used in the fields of immune antibody/antigen biomolecules interaction, analytical tracing ions detection and label-free biomarker sensing. With advantages on material cost, simple fabrication, and competitive performance, it is believed that the LSPR based SAM-AuNIs sensors would be attractive to the global biosensing community.

Date of Award	4 May 2017
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Lawrence WU (Supervisor)