Flexible V ₂ O _{5- x} Nanobelts as SERS Tweezers: Defect Engineering for Remarkably Sensitive and Selective Detection

Transition metal oxide (TMO)-based semiconductor nanomaterials (NMs) have been progressively explored as promising substrates for surface-enhanced Raman scattering (SERS). The design and synthesis of highly stable, extremely sensitive, exceptionally selective SERS substrates, solely based on defects engineered in TMO-based NMs, have gained significant interest. This work demonstrates a simple hydrothermal route to synthesize substoichiometric V 2 O 5- x flexible nanobelts without the use of surfactants, stabilizing agents, reducing agents, or structure-directing agents. Furthermore, the V 2 O 5- x nanobelts proved to be highly effective as a SERS substrate for the sensitive detection of methylene blue (MB), achieving a remarkable maximum enhancement factor of up to 6.75 × 10 9 and a detection limit as low as the picomolar level. This performance is the best among metal oxide semiconductors and is comparable to that of noble metals, even without the presence of a “hot spot.” Additionally, the V 2 O 5- x nanobelts demonstrate excellent selectivity as a “SERS Tweezer,” enabling the precise detection of MB even in the presence of interfering analytes across binary, ternary, and quinary mixtures. Notably, V 2 O 5- x nanobelts successfully detected melamine and ethephon for the first time using a TMO-based SERS substrate. In addition, the detection limit for SERS using a V 2 O 5- x substrate reached 3 ppm of melamine in liquid milk. Thus, our results clearly demonstrate that metal oxide semiconductors can be transformed into cost-effective, SERS-active substrates through defect engineering. These engineered substrates exhibit high sensitivity, selectivity, stability, recyclability, and biocompatibility─comparable to or even surpassing those of noble metal nanomaterial-based SERS substrates. This advancement could enable their effective use in detecting contaminants in foods and food ingredients.