Photodirector Ultra

[Rodney Liuzzo]

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Surface-enhanced Raman spectroscopy is one of the most sensitive spectroscopic techniques available, with single-molecule detection possible on a range of noble-metal substrates. It is widely used to detect molecules that have a strong Raman response at very low concentrations. Here we present photo-induced-enhanced Raman spectroscopy, where the combination of plasmonic nanoparticles with a photo-activated substrate gives rise to large signal enhancement (an order of magnitude) for a wide range of small molecules, even those with a typically low Raman cross-section. We show that the induced chemical enhancement is due to increased electron density at the noble-metal nanoparticles, and demonstrate the universality of this system with explosives, biomolecules and organic dyes, at trace levels. Our substrates are also easy to fabricate, self-cleaning and reusable.

The two key contributions to the SERS enhancement are the electromagnetic (EM) factor and the chemical contribution9. The EM enhancement of SERS-active substrates is by far the most important, and it is mainly determined by the nanostructure of the metallic surface and the wavelength-dependent dielectric properties of the metal. The conduction electrons of these nanoscale features can be driven by the incident electric field in collective oscillations known as localized surface plasmon resonances (LSPRs). These plasmonic materials are able to localize the electromagnetic field at sub-wavelength scales, thus effectively acting as nano-antennas10,11,12,13. However, the difficulty in arranging nanoparticles on a substrate to create the ideal SERS hotspots, means that the possible sensitivities are often not obtained in an easy or repeatable fashion14. On the other hand, the chemical contribution has been associated with smaller additional enhancements and its microscopic origin and overall contribution to the SERS enhancement are still under debate. Several models have been put forward depending on the specific system under study, including the adatom15, the metal-molecule charge transfer resonance9,16,17 and the polarizability modulation models18,19. These models were proposed to explain enhancements in different systems, but there are few demonstrations on how to induce and generalize this mechanism/effect.

SERS on (non photo-activated) semiconductor materials has been explored20,21,22. It has been shown that the main enhancement mechanism is non-plasmon based23,24, with enhancement factors, until recently, lower than those reported for metallic substrates25. The incorporation of metallic particles onto these (non-active) substrates leads to hot-electron migration from the particles to the semiconductor under visible light illumination26, with no effect on the SERS signal beyond that arising from the metallic particles alone27. There has also been limited work on the incorporation of semiconductor nanoparticles on roughened gold SERS substrates for enhancement28.

To further demonstrate the effect, the reduction of PIERS intensity after standing the substrate in the dark was monitored over an hour, in comparison to a non-irradiated SERS film, to show how recombination of the TiO2 excitons reduced the enhancement (Fig. 3c). From Fig. 3b and c, we can estimate the initial injected charge density (at time 0) is about 10%. Finally, a similar substrate was prepared using a more photo-inactive SiO2 film with similar AuNP and analyte loadings, and no significant PIERS enhancement was observed after the ultraviolet pre-treatment (Supplementary Fig. 5); nor was any enhancement observed in the absence of metallic nanoparticles (Supplementary Fig. 6).

The (average) order of magnitude enhancement found for PIERS respect to SERS and the inhomogeneous enhancements between different peaks in the Raman spectra when comparing both, points towards an improved chemical enhancement in PIERS, beyond the typical EM effect39. Indeed, in both AuNP and AgNP cases, the injected charges will shift the Fermi level of the nanoparticle to more negative potentials. The exact value of the new Fermi level is dependent on the size of the nanoparticle40, and as a consequence we expect a broad distribution of Fermi level values depending on both the intrinsic size of the nanoparticle and the amount of charge injected. This fact is also reflected in the spectral broadening of the post-irradiated sample in Fig. 3b. Having isolated particles on the substrate avoids equilibration of the charges (Fig. 1), resulting in different individual contributions to the observed chemical enhancement in the PIERS signal, depending on each particular Fermi level shift.

To account for the influence of a broad and new AuNP Fermi level distribution in the PIERS signal (compared to the SERS signal), it is necessary to analyse the parameters involved in the molecule-to-substrate (or vice-versa) charge transfer process, (the origin of the chemical enhancement)16. In this way, the differential Raman cross-section in SERS can account for the microscopic contributions to the chemical enhancement mechanism. By computing this parameter, Tognalli et al.9 clearly show how depositing a single Ag layer over an active Au SERS substrate results in the appearance of a new molecule Fermi level charge transfer resonance, thus enhancing the Raman signal of 4-mercaptopyridine molecules. In our case, the injected charges also shift the Fermi level of the AuNP over a broad distribution of more negative values. As a consequence, this will broaden the resonance conditions between the Fermi level and the molecular orbitals, increasing the charge transfer transitions probabilities over a wider variety of molecules deposited on the substrates.

The possibility of photo-inducing chemical enhancement of SERS, independently of the nature of the molecule, even for low Raman cross-section species, leads to a wide range of applications for this PIERS technique. An area where SERS substrates are of great value is in homeland security, for detection of high explosives during environmental monitoring and post-blast forensics41. This requires SERS techniques that work on inexpensive, reusable substrates with high sensitivities for low-cross-section molecules such as nitro-aliphatic PETN (pentaerythritol tetranitrate) and RDX (cyclotrimethylenetrinitramine), as well as the widely used TNT (trinitrotoluene) and its decomposition product DNT (dinitrotoluene). Other areas of interest are small biomolecule sensing, for example in glucose monitoring, or pollution monitoring and control, with similar substrate requirements.

To demonstrate the versatility of the PIERS beyond DNT and R6G, other materials of interest with low Raman cross-sections, such as pentaerythritol tetranitrate (PETN), RDX (cyclotrimethylenetrinitramine) (explosives) and glucose (biomarker) were tested (Fig. 4; Supplementary Fig. 10). There was significant enhancement over their solid and SERS spectra, demonstrating the power of the PIERS technique to enhance spectra of many classes of explosive molecules, not just high cross-section organic dyes or nitroaromatics.

We would like to thank Dr Steven Firth for useful discussion on the Raman spectra. SBJ acknowledges the support of the government of Saudi Arabia, Ministry of Interior, King Fahd Security College (KFSC), WJP is supported by EPSRC Doctoral Prize Fellowship EP/M506448/1, EC is supported by a Marie Curie fellowship. SAM and EC acknowledge the EPSRC Reactive Plasmonics project EP/M013812/1, the Office of Naval Research, the Royal Society, and the Lee-Lucas Chair in Physics.

Experiments were designed by S.B-J., W.J.P., R.Q-C., E.C. and I.P.P. Materials synthesis was performed by W.J.P., C.S-V., E.C. and S.B-J. Raman experiments were performed by S.B-J. and R.Q-C., materials characterization by C.S-V., W.J.P. and R.Q-C., and plasmonic measurements by E.C. Explosives samples were provided, prepared and handled by N.A-K. Data analysis and interpretation was performed by S.B-J., W.J.P., R.Q-C., E.C., S.A.M. and I.P.P. The manuscript was written by W.J.P., R.Q-C., E.C., S.A.M. and I.P.P., with input from all the authors.

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