ZnO and TiO2nanostructures for surface-enhanced Raman scattering-based bio-sensing: A review

Applications of Raman spectroscopy in bio-sensing and bio-detection were previously limited due to its low sensitivity resulting from weak Raman scattering. Over time, improvements in instrumentation and the discovery of surface-enhanced Raman scattering (SERS) have increased sensitivity and specificity for target analytes, making Raman spectroscopy a technique of choice for many applications, ranging from biomedical to environmental sensing. However, despite significant progress in the field, novel SERS substrates are still being synthesized and tested by researchers to achieve lower detection limits and increase the reproducibility of this technique. Accordingly, substrate materials have developed from traditional plasmonic materials, which provide primarily electromagnetic enhancements, to semiconductor materials that primarily rely on the chemical enhancement mechanism. A diverse body of research has equally explored combining these materials in a hybrid manner with variable results. Notable semiconductor materials include metal oxides ZnO and TiO2, which exhibit high flexibility and integration into SERS sensing designs to potentially enhance sensitivity and modify biocompatibility. In light of these recent developments, this review summarizes recent approaches to synthesize ZnO and TiO2SERS nanostructures and examines recent biological applications of these substrates. Over 60 papers were consulted to develop an overview of current synthesis approaches to integrating ZnO and TiO2into SERS substrates. A summary of various bio-detection applications and potential advancements in the field are presented.