Surface‐enhanced micro‐CARS mapping of a nanostructured cerium dioxide/aluminum film surface with gold nanoparticle‐bound organic molecules

Abstract

Highly contrast epi‐surface‐enhanced coherent anti‐Stokes Raman scattering (SECARS) microimages of Au‐nanoparticle‐bound organic reporter molecule distributions at a surface of novel surface‐enhanced Raman scattering (SERS)‐active metamaterial junctions, based on nanoparticles spread over a nanostructured CeO2 faceted dielectric film, deposited on an Al sublayer, were recorded at two‐color picosecond excitation of the surface in the near‐infrared spectral range. For this, a scanning confocal laser‐based micro‐CARS spectrometer was employed. The investigations showed that at Raman resonant laser excitation of the molecules/Au‐NP conjugates immobilized on the surface strong SECARS signals can be generated with laser powers not deteriorating the conjugates. Coupling CARS with the plasmonic metamaterial structures under investigation provided excellent chemical imaging contrast (up to 400) for biochemically relevant 5‐thio(2‐nitrobenzoic acid) and 4‐mercaptophenylboronic acid reporter molecules. Taking into account easy handling and utmost long‐term stability of the investigated metamaterial junction at ambient conditions, it might be considered as a promising perspective for a single‐molecule‐sensitivity surface‐enhanced Raman scattering or SECARS biosensor.

Laser intensity limits in surface‐enhanced linear and nonlinear Raman micro‐spectroscopy of organic molecule/Au‐nanoparticle conjugates

“…The initial steep slope of the P aS versus ( P p 2 × P S ) dependences in the bright spots is evaluated to be ≈0.003 counts/μW 3 . This value correlates with the P p 2 × P S product‐normalized peak SECARS signal, presumably generated by TNB/Au NP conjugates, which had been estimated previously to reach ≈0.13 counts/μW 3 . The ~40 times distinction between the normalized SECARS signal strengths presented in this work and those from the study of Fabelinsky et al can be ascribed to the difference in the Au NP surface densities specific for the investigated samples.…”

“…In contrast, originally, "dark" spots (e.g., at [ X = 18, Y = 9] and [ X = 21, Y = 2]) still demonstrate noticeable growth of SECARS signal strength, and even nearly linear dependences of the signals (like at [ X = 10, Y = 17], but with somewhat different slopes) on the product P p 2 × P S . The slope of these dependences is ~(0.1–0.2) × 10 −3 counts/μW 3 and rather corresponds to the level of Raman nonresonant signals defined in the study of Fabelinsky et al as ~4 × 10 −3 counts/μW 3 . These signals result, presumably, from two‐color four‐wave mixing processes near one‐ or two‐photon absorption resonances within various unmodified Au NP structures immobilized on the CeO 2 film surface.…”

“…Spectra and microimages of SECARS and SERS radiation were recorded, with a high spatial resolution, using a Confotec CARS confocal CARS Raman microspectrometer (SOL Instruments Ltd., Belarus), described in detail in the study of Fabelinsky et al The CARS part of the spectrometer was based on a diode‐pumped passively mode‐locked Nd 3+ :YVO 4 laser (1064 nm, 7 ps, 85 MHz, 5 W, ~5–7 cm −1 linewidth) and an optical parametric oscillator (OPO) synchronously pumped by the frequency‐doubled 1,064‐nm radiation (2 W). The signal beam of the OPO (690–990 nm, 6 ps, 150–350 mW,) and a part of the 1,064‐nm beam, with parallel polarizations, were collinearly overlapped and employed, respectively, as pump ( λ p ) and Stokes ( λ S ) radiation in the CARS process to generate narrow lines of the coherent anti‐Stokes signals at λ aS = λ p /(2 – λ p / λ S ).…”

“…Within such an approach, a randomly nanostructured cerium dioxide‐faceted dielectric film, deposited on a thin aluminum sublayer at an aluminum oxide ceramic substrate (CeO 2 /Al/Al 2 O 3 ), has been recently reported to show surface enhancement of spontaneous Raman scattering . The first experiments with SERS and SECARS by 5‐thio(2‐nitrobenzoic acid) (TNB)/Au NP conjugates, rarely spread over the surface of the CeO 2 film, have shown a feasibility to obtain signals with high spatial contrasts ("bright spots").…”

Development and applications of nonlinear optical spectroscopy: 16th ECONOS/36th ECW meeting in Jena (Germany)

“…This led to the conclusion that SRS can be an efficient method for monitoring microplastics in the environment. Fabelinski et al demonstrate epi‐surface‐enhanced coherent anti‐Stokes Raman scattering (SECARS) microimaging of Au‐nanoparticle‐bound organic reporter molecules. Their method utilizes surface‐enhanced Raman scattering (SERS)‐active metamaterial junctions, which are based on nanoparticles spread over a nanostructured CeO 2 faceted dielectric film, deposited on an Al sublayer.…”