Preparation And SERS Sensing Applications Of Graphene-interlayered Nanocomposites Based On Noble-metallic And Magnetic Nanoparticles

Ultrasensitive detection and characterizaion of targeted analytes is crucial in various fields,such as biological sensing,analytical chemistry,environmental science,and in vivo biomedical studies.General methods,like those utilizing titrimetry,fluorescence and spectrophotometer,have disadvantages of high cost and low productivity.Surface enhanced Raman scattering?SERS?overcomes these drawbacks,and attract wide interests due to its high sensitivity,rapid and accurate detection.However,compared with other methods,SERS signals are much weaker,which need efficient SERS substrates to provide enough augment.The sensitivity of SERS substrates are the most important aspect in consideration,while other properties,such as stability,reproducibility,cost and compatibility,are crucial as well in practical applications.The major materials utilized for fabricating SERS substrates are noble metals,and researchers proposed many well-designed nanostructures with a destination to obtain “hot spots” according to the electromagnetic enhancement theory.Studies about the fabrication protocol and structure design of SERS substrates always are the most concerning issues.Herein,we think from the point of proposing a better fabrication protocol and achieving more superior properties.We improved the protocol of preparing Fe₃O₄@Ag,and introduced graphene in order to achieve a better performed SERS substrate.The main results are concluded as follows:1.We studied some reported methods about preparing Fe₃O₄@Ag,and evaluated their performances,and then proposed a novel strategy,PEI-mediated seeds reduction method for improved growth of Fe₃O₄@Ag,which is suitable for magnetic nanoparticles with various diameters,ranging from 100 nm to 800 nm.The PEI polymer wrapped on the surface of Fe₃O₄ nanoparticles,and modified it with abundant amino-groups.The 2 nm PEI layer brought in dual benefits due to its excellent hydrophilic properties,including improved dispersion and facile adhesion of 3 nm Au NPs on the surface.These Au NPs acted as seeds for further growth of silver shell.Specifically,after adding AgNO3 and reductant,small Ag nanoparticles will be formed first and then grew into a compact silver shell.All fabricated nanoparticles are examined by various methods for characterization of related size,morphology and component.Different analytic results consistently declared that the prepared nanocomposites possess narrow size distribution and high reproducibility.The SERS performance was evaluated by detecting a standard Raman probe PATP and a protein molecule IgG,and the respective limit of detection is 10-11 M and 10-14 g mL-1.Both demonstrated that the silver-shelled magnetic nanoparticles,Fe₃O₄@Ag,is a potential high performed SERS substrates,and possess well structure and properties.2.We studied the weaknesses existing in those reported graphene-mental nanocomposites,and proposed a new strategy,which utilizing the novel versatile 2D material,graphene,to combine with silver-shelled magnetic nanoparticles for fabrication of multifunctional SERS substrates.The designed structure Fe₃O₄@Ag-rGO-Au rationally balanced the respective excellent properties of magnetic Fe₃O₄,Ag,and multifunctional graphene.The characterization of sample's structure and morphology declared the successful preparation of proposed novel magnetic nanocomposites.And the introduction of graphene oxide improved the dispersibility of Fe₃O₄@Ag dramatically.Similarly,the specific SERS performance of nanocomposites is examined though detection of PATP and DTTC Raman probe molecules,whose limit of detection are 10-11 M and 10-10 M,respectively.3.We examined the SERS performance of Fe₃O₄@Ag,Fe₃O₄@Ag-GO and Fe₃O₄@Ag-rGO-Au,and accordingly further studied the specific contributions to SERS signal of silver shell,graphene and Au NPs.Through comparing their Raman characterized peaks,and analyzing the differences of the Raman shifts and peak intensities,we discussed that graphene has considerable chemical enhancement ability and decent absorption of analytes.As reported by other researchers,there are plasma coupling resonances between Au NPs and the Ag shell,which produced outstanding electromagnetic enhancement.