Harnessing Octadecyl Trimethylammonium Bromide Stabilized Gold Nanorods as a Sensitive Visual Detection Platform: Detection of p-Aminophenol at nM Levels as a Case

Gold nanorods (AuNRs), as versatile sensing materials, have wide analytical applications due to their unique optical properties. Cetyltrimethylammonium bromide (C₁₆TAB), a conventional reagent in AuNR synthesis, also often acts as a stabilizer of AuNRs in applications. However, C₁₆TAB-stabilized AuNRs undergo severe spontaneous aggregation and etching under extreme pH conditions, greatly limiting their optical sensing applications. Herein, we accidentally discovered that octadecyl trimethylammonium bromide (C₁₈TAB), a rarely used surfactant for AuNRs, has a substantially higher stabilizing ability than C₁₆TAB in preventing spontaneous aggregation and etching of AuNRs, which enables C₁₈TAB-stabilized AuNRs as a superior sensing platform, demonstrating a 100-fold higher sensitivity than C₁₆TAB-stabilized AuNRs for detection of model analytes. The excellent stability of C₁₈TAB-stabilized AuNRs can be attributed to the higher surfactant coverage density on the gold surface, evidenced by the red-shifted longitudinal band (5 nm), which is tuned by the metal surface refraction index. The experimental results show that C₁₈TAB-stabilized AuNRs can keep monodispersed and unchanged optical properties at very acidic and alkaline conditions with a low concentration of surfactant (0.05 mM). Moreover, the C₁₈TAB-stabilized AuNRs can prevent spontaneous etching in the acidic sensing system and maintain their unchanged plasmon band, therefore decreasing the intensity of the noise signal. Benefiting from these findings, we established a reliable and ultrasensitive C₁₈TAB-stabilized AuNR sensing platform and achieved the ultrasensitive detection of the model biomarker p-aminophenol (pAP), with a visual detection limit of 8 nM. This sensitivity represents at least a 100-fold improvement over the existing method using C₁₆TAB-stabilized AuNRs. Moreover, C₁₈TAB-stabilized AuNRs were successfully applied to detect pAP in urine samples with satisfactory recovery rates of 99.84-114.91%, further validating its reliability in practical applications. In summary, C₁₈TAB-stabilized AuNRs provide a powerful tool for trace-level visual detection in chemo- and biosensing.