Pressure-Induced SERS-Enhanced Sensing of 4-Mercaptobenzoic Acid Using Few-Layer WS2 Nanosheets

Pressure-induced surface-enhanced Raman spectroscopy (PI-SERS) is among the foremost achievements of high-pressure fields, benefiting from the advantage that pressure-modulated interactions between substrates and molecules overcome the weakness of individual substrates or molecules. However, the pressure corresponding to PI-SERS enhancement is currently still at a low-pressure stage within 8 GPa. Therefore, how to realize PI-SERS enhancement under higher or ultrahigh pressure remains a huge challenge. Here, we applied few-layer WS2 (FL-WS2) nanosheets as PI-SERS substrates and selected 4-mercaptobenzoic acid (4-MBA) molecules with a big energy gap as probes to explore PI-SERS enhancement in more depth. The experimental results indicate that the PI-SERS signals increase gradually as pressure increases and reaches a maximum at 18 GPa, which has set a pressure record for the PI-SERS field so far. Different from the C exciton resonance-induced SERS intensities at ambient conditions, the charge-transfer transition between FL-WS2 and 4-MBA is strengthened via the pressure-induced increase in the band gap of FL-WS2 and decrease in the highest-occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) energy gap of 4-MBA, thus enhancing PI-SERS signals accordingly. Therefore, this strategy will be beneficial for practical applications in the field of optoelectronics and for the design of other PI-SERS sensors for trace detection in complex and extreme environments.