SERS Performance Factor: A Convenient Parameter for the Enhancement Evaluation of SERS Substrates

Surface-enhanced Raman spectroscopy (SERS), with molecular fingerprint information and single-molecule sensitivity, has been widely used for qualitative and quantitative analysis in various fields. Plenty of nanostructured plasmonic materials have been fabricated to achieve high SERS activity. Currently, great difficulty lies in evaluating the SERS performance among substrates, making it difficult to standardize. Addressing this problem, this work proposed the SERS performance factor (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>S</mml:mi><mml:mi>P</mml:mi><mml:mi>F</mml:mi><mml:mo>=</mml:mo><mml:mfrac><mml:mrow><mml:mi>Δ</mml:mi><mml:msub><mml:mrow><mml:mi>I</mml:mi></mml:mrow><mml:mrow><mml:mi>S</mml:mi><mml:mi>E</mml:mi><mml:mi>R</mml:mi><mml:mi>S</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:mi>Δ</mml:mi><mml:msub><mml:mrow><mml:mi>C</mml:mi></mml:mrow><mml:mrow><mml:mi>S</mml:mi><mml:mi>E</mml:mi><mml:mi>R</mml:mi><mml:mi>S</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:mfrac><mml:mo>/</mml:mo><mml:mfrac><mml:mrow><mml:mi>Δ</mml:mi><mml:msub><mml:mrow><mml:mi>I</mml:mi></mml:mrow><mml:mrow><mml:mi>R</mml:mi><mml:mi>a</mml:mi><mml:mi>m</mml:mi><mml:mi>a</mml:mi><mml:mi>n</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:mi>Δ</mml:mi><mml:msub><mml:mrow><mml:mi>C</mml:mi></mml:mrow><mml:mrow><mml:mi>R</mml:mi><mml:mi>a</mml:mi><mml:mi>m</mml:mi><mml:mi>a</mml:mi><mml:mi>n</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:mfrac></mml:math>) as a practically operational parameter to evaluate the sensitivity of SERS substrates. Experimentally, <i>SPF</i> can be obtained by taking the ratio of the slopes (i.e., the sensitivity) for concentration-dependent SERS and normal Raman measurements in the linear range of the intensity response under identical experimental conditions. Theoretically, <i>SPF</i> quantitatively describes the overall contribution to the SERS performance, (i.e., the electromagnetic (EM) enhancement of the SERS substrate and the interfacial interaction between the probe and substrate). The use of <i>SPF</i> as the criterion for evaluating the SERS performance was validated on Au nanoparticles in colloidal and solid states, where the tendency of <i>SPF</i> is consistent with that of the sensitivity of the probe molecules. Derived from the typically used surface enhancement factor EF in which accurate parameters are hardly achievable and different from concentration-dependent analytical enhancement factor AEF, <i>SPF</i> distinguishes itself with a simpler calculation and thereby offers a convenient and reliable protocol for the evaluation of the performance of different SERS substrates, which is very important to the practical application of SERS.