Semiconductor-enhanced Raman Analysis Based On Charge Transfer Mechanism

SERS,or surface-enhanced Raman spectroscopy,is the significant enhancement of Raman spectra when molecules adsorb onto rough noble metal surfaces.For analytical applications,SERS is a highly sensitive analytical technique that provides rich vibrational spectrum information,serving many different applications,including electrochemistry,catalysis,biology,medicine,art protection,material science,etc.At present,noble metal SERS substrates based on the electromagnetic mechanism（EM）show ultra-high Raman enhancement(10¹²),which can be analyzed at the single-molecule level;however,the EM-based SERS effect has no selectivity for Raman reporters,resulting in complicated signal outputs,which cannot meet the high specificity and high accuracy requirements of trace molecular analysis.Another SERS mechanism is chemical mechanism（CM）,which utilize the charge transfer（CT）processes between adsorbed molecules and SERS substrate,so the selectivity and signal stability are greatly improved.Unfortunately,the CM-based SERS effect shows relatively low EFs（10³-10⁵）since the CT process is a short-range one.In recent years,semiconductor nanomaterials have become potential CM-based SERS substrates due to their excellent SERS activity.Therefore,it is urgent to develop semiconductor nanomaterials with high SERS activity and analyze their chemical enhancement mechanism in detail,to achieve selective multi-channel detection,so that the application of SERS technology in complex system analysis can be further improved.This thesis has addressed the key scientific issues raised above by carrying out the following three aspects of work:（1）In this study,WO₃-Sn O₂ nanoflake arrays were synthesized on FTO by hydrothermal synthesis and pulse electrodeposition,revealing significant SERS activity with an EF reaching 4.79×10⁷.The obvious EF was mainly ascribed to the charge transfer between WO₃-Sn O₂ and MB based on CM and the molecular resonance effect.With the competitive adsorption of DA and absorbed MB,we prepared a SERS and EC dual-mode detection platform of DA based on the WO₃-Sn O₂ nanoflake arrays.The linear ranges were 5.00-1.80×10³ nmol/L and 2.50-1.75×10³ nmol/L,as well as the detection limits（LODs）were as low as 1.50 nmol/L and 0.80 nmol/L by SERS and EC respectively.Besides,the developed detection platform can shield the interference of many neurotransmitters similar to DA,showing good selectivity and excellent stability.In general,the SERS-EC dual-mode detection platform can be well applied to the detection of DA in cell lysate,demonstrating great potential in diagnosis of neurodegenerative diseases.（2）Here,a novel sponge-like Cu-doping Sn O₂-Ni O p-n semiconductor heterostructure（Sn O₂-Ni O_x/Cu）,was first created as a CM-based SERS substrate with a significant EF of 1.46×10¹⁰.This remarkable EF was mainly attributed to the enhanced charge-separation efficacy of p-n heterojunction and charge transfer resonance resulted from Cu doping.Moreover,the porous structure enriched the probe molecules,resulting in further SERS signals magnification.By immobilizing Cu Pc as an inner-reference element,Sn O₂-Ni O_x/Cu was developed as a SERS nose for selective recognition of multiple lung cancer related VOCs down to ppb level.The information of VOCs was recorded in a barcode,demonstrating practical potential of a desktop SERS device for biomarker screening.（3）In this work,we developed a novel Ga-doped Zn O（GZO）superlattice nanocube as SERS substrate by hydrothermal method combined with calcination synthesis,and the EF value reached 1.01×10¹¹.Both the experimental results and theoretical calculation show that the SERS enhancement is mainly due to the superlattice structure that forms a periodic sequence of electrons-and holes-filled quantum wells,effectively inhibiting the recombination of charges and holes and improving the space charge separation efficiency;In addition,the surface defects at the superlattice interface make electron migration need to overcome a lower energy barrier,which accelerates the charge transport across the interlayer oxide barrier.Both of them together lead to efficient charge transfer.With the development of follow-up experiments,the phenomenon of charge transfer between molecules and materials will be better demonstrated.Furthermore,we used the GZO superlattice SERS substrate to detect four epilepsy-related substances(S100β,Cu²⁺,Ca²⁺and O₂^·-),realized sensitive and accurate analysis in cells,and speculated on the interaction of the four substances.These studies not only greatly help the preparation and mechanism research of semiconductor SERS substrates,but also provide new ideas for the analysis and application of semiconductor SERS nanomaterials in various complex systems.