Design And Applications Of Highly Sensitive SERS Chip Based On Accessibility Of "Hot Spots"

Localized surface plasmon resonances(LSPRs)is able to utilize plasmonic nanostructures to concentrate incident electromagnetic(EM)fields at highly spatially localized regions,leading to remarkable enhancement of local fields and allowing for manipulation of light below the diffraction limit.Especially when two metal nanoparticles(NPs)close each other,due to plasmon coupling,EM field at the nanogap is enhenced hundreds of times,known as the plasmonic “hot spots” effect.Due to the EM enhancement of LSPR,molecules around plasmonic nanostructures produce Raman signals 106 times stronger than in the free state,and this effect is surfaceenhanced Raman scattering(SERS).As a molecular vibrational fingerprint spectrum,high sensitive SERS analytical technique has been widely applied in physics,chemistry,biology,materials and other fields.Bottom-up methods of preparing SERS substrates are widely used because they are usurally low cost and can be combined with various synthesis strategies of nanoparticles(NPs)to prepare various SERS substrates with tips,gaps and slits.However,there are two problems which restrict the practical application of SERS substrates.First,random distribution of NPs causes nonuniform SERS signal.Second,it is difficult for analytes to enter tiny nanogaps due to steric hindrance or preoccupation by ligands and impurities,which limits the sensitivity of SERS detection.Therefore,efficient SERS measurements require not only uniform metal nanostructures with very narrow gaps,but also these “hot spots”are accessible for target molecules.In this dissertation,we developed new strategies to realize the goal of preparing SERS substrates with uniform,strong and effective “hot spots”.We built highly sensitive SERS chip and Lo C-SERS chip,and explored their application in the field of trace pollution detection,insitu study of protein conformational change and plasmon-mediated chemical reactions(PMCR).The main contributions of this dissertation are list as follow:(1)The uniform and size controllable near-spherical silver nanoparticles(Ag NPs)were prepared with the proposed two-step method.Ag NPs with 65 nm diameter were evenly coated on hydrophobic PDMS by optimized layer-by-layer assembly protocol to form SERS substrates with uniform distribution of “hot spots”.A SERS-active microchannel was fabricated by integrating the SERS-active substrate in a PDMS microchannel.The SERS active microchannel is more suitable to study native protein rather than dried solide SERS substrates.(2)Based on thermal shrinking property of PVC sheets,SERS substrates with adjustable nanogaps were fabricated.Two SERS working modes,adsorption-shrink and shrinkadsorption,were studied.It was found that due to the steric hindrance,SERS intensity in the shrink-adsorption mode presented a complex pattern with reduced nanogaps.The adsorption-shrink mode can exclude the steric hindrance.The results show that effective transporting analytes to “hot spots” is one of the prerequisite conditions to realize ultrasensitive SERS detection.The concept of “hottest zone” was proposed to quantify contributions of analytes number and gap size to SERS enhancement.(3)The adsorption-shrink mode,which excluds the steric hindrance,was proved to achieve single-molecule detection.Three trace pollutants,thiram,malachite green(MG),and formaldehyde,which adsorbed on Ag NPs by chemical bonds,electrostatic interactions,and no metal affinity,were detected with the adsorption-shrink mode and limits of detection(LOD)are much lower than domestic and international standards.Therefore,the adsorption-shrink SERS chip could be a gereral method in the detection of trace pollutants.(4)An ultrafast mixer-assisted “hot spots” occupying(MAHSO)SERS strategy was developed to accomplish highly sensitive in-situ studies on the premise of maintaining activities of biological macromolecules.Because adsorption of analytes on metal NPs occurs before NPs deposit on supports,analytes are naturally located in “hot spots”,the gaps between adjacent NPs.The MAHSO SERS chip was used to conduct in situ study on conformational changes of proteins under different p H conditions.(5)The plasmon-mediated chemical reactions were investigated in situ by using MAHSO SERS method.The oxidation of p-aminothiophenol(p ATP)was used as the model reaction.It was found that the reaction in the “hot spots” regions is directly driven by hot carriers and shows higher reaction rate.Reaction paths are also different with those in “non-hot spots” regions.These findings have experimentally proved that hot carriers generated from “hot spots” with higher energy and points out that rational design of photocatalysts should consider the utilization of “hot spots”.