Preparation Of Magnetic Polyethyleneimine-based SERS Substrates And Their Applications In Sensitive And Specific Detection

Surface enhanced Raman spectroscopy(SERS)technology is widely used in food,environment,biological and other detection fields due to its advantages such as simple operation,fast speed and high sensitivity.An ideal SERS sensor needs to have excellent sensitivity,stability,selectivity and specificity.As a sensor,sensitivity has always been a key factor.Besides,stability of the existing SERS substrates is not optimistic because of the easy oxidation and agglomeration of precious metal nanoparticles themselves.In addition,selectivity and specificity are also challenges.Sensors are required to eliminate interference in actual samples as much as possible to achieve the detection of target analysts.These three indexes are the key to construction of a SERS sensor.In view of the above,this work is also dedicated to constructing SERS substrates with high sensitivity,stability,and specificity.Here,polyethyleneimine(PEI)modified magnetic materials are used to improve stability and sensitivity of SERS substrate.They often function as adsorbents,which can enrich analytes and increase detection sensitivity.On the other hand,PEI modified magnetic materials can immobilize noble metal nanoparticles and reduce agglomeration of them,thus enhencing the stability of SERS substrate.Combination with molecular imprinting technique is a effective strategy for specificity of the SERS sensor.The specific cavities of MIP are used to specifically capture target molecules,thereby improving specificity of SERS detection.Based on this,two aspects of work have been carried out to improve sensitivity,stability and specific performance.(1)A simple self-assembly method is reported to fabricate magnetic Fe3O4@PEI@Ag SERS substrate.Ag nanoparticles(AgNPs)assembled on Fe3O4NPs help form more SERS hotspots by magnetic induction,which ensures direct aqueous detection and high sensitivity(EF=4.0×106 in wet state)of Fe3O4?PEI@Ag sensor.PEI not only acts as an interlayer for assembling AgNPs to Fe3O4 particles,but can also direct the interaction between tetracycline(TC)and SERS substrate.The binding ratio between B,C,D rings of TC and PEI is calculated to be 1:4 by Ultraviolet-visible spectroscopy(UV-vis)characterization.Simulation study also confirms the multiple TC-PEI interaction modes(hydrogen,?-?,cation-?).Fe3O4@PEI@Ag sensor also shows very high stability(signal decreases<5%after 93 days).Fe3O4@PEI@Ag SERS sensing array could have a great potential for real-time sensitive detection of analytes.More importantly,this rational adjustment of interaction sites between substrate and analyte is expected to be a new strategy for the improvement of SERS detection sensitivity and selectivity.(2)Based on the previous work,molecular imprinting was introduced to improve specificity of SERS substrate.The PEI-based magnetic material in previous work is promising for preparation of core-shell MIP.Directly cross-linking PEI reduces steps of monomer-initiated polymerization and monomer washing,and simplifies fabrication process.On the other hand,strong adsorption capacity and hydrophilicity of PEI contribute to better imprinting and rebinding of template molecules.Through magnetic enrichment and strong adsorption capacity of magnetic PEI material and the specific adsorption of molecular imprinting,specific and sensitive SERS detection of vanillylmandelic acid(VMA)was achieved.The linear quantitative range of this detection method in the wet state is 10-6 to 50×10-6 mol/L,which meets determination in urine samples.The investigation of dynamic adsorption performance found that the magnetic PEI material has strong adsorption performance,equilibrium adsorption amount reaches 42.72 ?mol·g-1.Also,its adsorption selectivity is good.The same method was used to fabricate methylene blue-imprinted polymers,which can also be used for SERS detection.