The Synthesis, Optical Properties and Immunoassay Applications of Gap Enhanced Raman Tags (GERTs)

Author:Lin Li

Supervisor:gu hong chen ye jian


Degree Year:2018





The optically labeled immunoassay is a widely used detection technique,for which a variety of optical markers are used to amplify the signal of antigen-antibody reaction,and to achieve a convenient and sensitive detection.Among various optical markers employed for immunoassays,surface enhanced Raman spectroscopy(SERS)-based markers has gained much attentions in recent decades.The SERS-based markers,usually named as SERS probes,are consisted of metallic nanoparticles(NPs),Raman molecules adsorbed on NPs,protective layers,and targeting biomolecules.For a SERS probe,the metallic NP substrate plays a key role.The NPs with ultra-small gaps show large Raman enhancement.It is of much importance to prepare SERS substrates with high signal enhancement,good reproducibility and stability.Recently an increasing number of publications have reported the core-shell structured metallic NPs with internal nanogaps.The Raman molecules are embedded in the internal gaps,thus improving the intensity,reproducibility,and stability of the Raman signal,making these particles excellent SERS probes.At present,these core-shell SERS probes have been applied in various multiplex immunoassays.In this dissertation,the core-shell structured metallic NPs with internal gaps,named as gap enhanced Raman tags(GERTs)is studied.The research includes the chemical synthesis,signal enhancement physical mechanism and related immunoassay applications of GERTs.In Chapter 2,we investigate the surface morphology and optical properties of GERTs,confirming that they have strong and reproducible SERS signals.Through the transmission electron microscopy(TEM)characterization of the reaction intermediates,we study the Au shell growth process and nanogap formation mechanism of GERTs.Detailed analysis show that the embedded molecular layer plays a key role in forming the gap structure,and the thickness of the molecular layer determines the gap width;the affinity to Au and the occupied area of the molecular layer on the Au core surface influence the integrity of the nanogap structure;as the coverage of the molecular layer alters,the "gap percentage" and Raman properties of GERTs also change.This work helps to understand the chemistry in synthesis and nanogap formation of GERTs.In Chapter 3,we experimentally and theoretically study the optical response of metallic core-shell particles with different widths of interior gaps(0.7 nm,15 nm and 100 nm).It is found that the electron transport effects become significant in GERTs with subnanometersized gaps,which shields the gap mode resonance.The extinction spectrum calculated by the quantum-corrected model(QCM)agrees well with the measured spectrum,confirming the charge transfer effects in the GERTs particles.The SERS measurement of GERTs embedded with conductive or non-conductive molecular layers also demonstrates the result.In Chapter 4,we further study the control in chemistry of GERTs particles.By changing the number of the Au shells,we investigate the alteration in Raman performance of GERTs.The multi-layered core-shell GERTs particles are successfully synthesized by the repeated seed-mediated growth.The double-layer core-shell GERT have the strongest SERS signal,and its outer Raman molecules layer contributes most of the SERS signal for the whole particle.The Raman peaks and intensities of these multi-layered core-shell GERTs are highly tunable.We briefly discuss their potential application in Raman-based encoding.In Chapter 5,we establish the aromatic thiolate molecule-based chemistry for synthesizing GERTs with ultra-small interior nanogap,and practical strategies for producing quantifiable and tunable SERS signals from the nanogaps.By varying the incubation time of Raman molecules and Au core,a multiple molecular layer structure is formed on Au surface through disulfide bonds.The GERTs particles with different gap widths are prepared.Furthermore,these GERTs with tunable internal gap width provide a platform to study the electron transport effect induced by the molecular layer.We used the SERS technique to study the near-field optical properties of GERTs,confirming that the enhanced Raman signals is influenced by the charge transfer effect in nanogaps of GERTs.The study possibly provides a way to optimize the Raman performances for the core-shell particles,as well as new insights for the development of quantum plasmonic devices.In Chapter 6,we briefly investigate the applicability of GERTs in immunoassay.The SERS-based lateral flow assay(LFA)strip are made using GERTs to achieve highly sensitive,quantitative and rapid detection of human chorionic gonadotropin(HCG),by analyzing the Raman signal on the LFA strip.The detection limit is two orders of magnitude lower than that of the commercial strips.This SERS-based LFA can be also measured using a portable hand-held Raman spectrometer,which facilitate its applications in more fields.In summary,this work focuses on the chemistry in the controllable synthesis of GERTs,interprets their nanooptics and physical mechanisms of signal enhancement,and briefly verifies their potential application in immunoassay.This research helps to understand the synthesis chemistry and optical properties of metallic core-shell nanoparticles,and provides a strategy to optimize their SERS performance,facilitating the SERS-tag based applications in biomedical immuno-detections.