AnisotropicallyShaped Plasmonic WO_3–<i>x</i> Nanostructure-DrivenUltrasensitive SERS Detectionand Machine Learning-Based Differentiation of Nitro-Explosives

Discovery of new surface-enhanced Raman spectroscopy (SERS) substrates consisting of inexpensive and earth-abundant elements is an unmet need for the advancement of future analysis techniques for the ultrasensitive detection and quantification of chemical and biological analytes. Nanostructures (NSs) of noble metals such as Au, Ag, and Cu are the benchmarks for the preparation of highly efficient SERS substrates because of their unique localized surface plasmon resonance (LSPR) properties. Non-noble-metal SERS substrates, e.g., metal chalcogenide semiconductors and transition metal oxides, have been prepared to mitigate the cost; however, their low sensitivity restricts widespread applications. In this article, we report for the first time that the structure of oxygen-deficient, LSPR-active, nonstoichiometric tungsten oxide (i.e., WO_3–<i>x</i>) NSs can control the SERS enhancement factor (EF). SERS substrates prepared with colloidally synthesized WO_3–<i>x</i> nanowires, nanorods, and nanoplatelets display SERS EF of 2.5 × 10⁶, 3.1 × 10⁷, and 5.5 × 10⁷, respectively, using rhodamine 6G (R6G) molecules as Raman probes. Our experimentally acquired SERS data and spectroscopically determined electronic band structure of LSPR-active WO_3–<i>x</i> NSs, and time-domain density functional theory (TDDFT)-based calculations support a dual enhancement scheme involving a strong plasmonic effect controlled electromagnetic field and their oxygen vacancy-induced chemical enhancement mechanisms, respectively. To demonstrate the practical utility of our WO_3–<i>x</i> NCs, we are able to detect aromatic nitro-explosives (tetryl, TNT, and DNT) with a very low limit of detection (LOD, 10^–9 M). Importantly, machine learning-driven chemometric analysis for SERS-based detection shows excellent classification between these three explosives. Finally, three nonaromatic, nitro-explosives, HMX, RDX, and PETN are also successfully detected utilizing our LSPR-active, WO_3–<i>x</i>-based SERS substrates. To the best of our knowledge, this is the first example where LSPR-active, non-noble-metal NSs are used for the detection of both aromatic and aliphatic nitro-explosives. Taken together, our work represents the advancement of the fabrication of non-noble-metal-based SERS substrates, which can be widely employed for the low-cost detection of analytes across forensic science, chemistry, and biomedical fields.