Surface-Enhanced Raman and Surface-Enhanced Hyper-Raman Scattering of Thiol-Functionalized Carotene

Abstract

A thiol-modified carotene, 7'-apo-7'-(4-mercaptomethylphenyl)-β-carotene, was used to obtain nonresonant surface-enhanced Raman scattering (SERS) spectra of carotene at an excitation wavelength of 1064 nm, which were compared with resonant SERS spectra at an excitation wavelength of 532 nm. These spectra and surface-enhanced hyper-Raman scattering (SEHRS) spectra of the functionalized carotene were compared with the spectra of nonmodified β-carotene. Using SERS, normal Raman, and SEHRS spectra, all obtained for the resonant case, the interaction of the carotene molecules with silver nanoparticles, as well as the influence of the resonance enhancement and the SERS enhancement on the spectra, were investigated. The interaction with the silver surface occurs for both functionalized and nonfunctionalized β-carotene, but only the stronger functionalization-induced interaction enables the acquisition of nonresonant SERS spectra of β-carotene at low concentrations. The resonant SEHRS and SERS spectra are very similar. Nevertheless, the SEHRS spectra contain additional bands of infrared-active modes of carotene. Increased contributions from bands that experience low resonance enhancement point to a strong interaction between silver nanoparticles and electronic levels of the molecules, thereby giving rise to a decrease in the resonance enhancement in SERS and SEHRS.

Figure 1

(a) Molecular structures of the two carotenoids examined in this study: 7′-apo-7′-(4-mercaptomethylphenyl)-β-carotene (1) and β-carotene (2). (b) Absorbance spectra of the silver nanoaggregates before and after the addition of carotene solutions with the same concentrations as in the SEHRS experiments. The inset in (b) shows a transmission electron micrograph of the silver nanoparticles (scale bar: 100 nm).

Figure 2

(a) SERS spectrum of 7′-apo-7′-(4-mercaptomethylphenyl)-β-carotene and (b) comparison between the SERS spectra of 7′-apo-7′-(4-mercaptomethylphenyl)-β-carotene (upper trace) and nonfunctionalized β-carotene (lower trace). Final concentration of the carotenes in the sample solutions: 3 × 10^–6 M; excitation: 1064 nm; peak photon flux density: 3 × 10²⁸ photons cm^–2 s^–1; and acquisition time: (a) 30 s and (b) 120 s. For the spectrum in (a), a total of 100 spectra from five different samples were averaged after baseline correction.

Figure 3

(a) SERRS spectra and (b) resonant Raman spectra of 7′-apo-7′-(4-mercaptomethylphenyl)-β-carotene (upper spectra) and β-carotene (lower spectra). Concentrations: (a) upper trace 3 × 10^–9 M; lower trace 3 × 10^–8 M; (b) both traces 3 × 10^–6 M. Excitation: 532 nm; 6 × 10²⁴ photons cm^–2 s^–1; 1 s acquisition time. Bands marked with an asterisk are due to the ethanol in the solvent.

Figure 4

SERS spectra of the carotenethiol 7′-apo-7′-(4-mercaptomethylphenyl)-β-carotene (3 × 10^–7 M, upper trace) and the nonfunctionalized β-carotene (10^–7 M, lower trace) with the spectrum from silver nanoaggregates without carotene molecules. The spectrum of the nonfunctionalized carotene was scaled up by a factor of 3 to facilitate comparison. Excitation: 514.5 nm, 2 × 10²³ photons cm^–2 s^–1, and 1 s acquisition time.

Figure 5

(a) SEHRS spectra excited at 1064 nm and (b) SERRS spectra excited at 532 nm of 7′-apo-7′-(4-mercaptomethylphenyl)-β-carotene (upper spectra in both panels) and of the nonfunctionalized β-carotene (lower spectra in both panels). Final concentrations of the carotenes in the sample solutions: 3 × 10^–7 M; excitation: (a) 1064 nm; peak photon flux density 6 × 10²⁸ photons cm^–2 s^–1; 120 s acquisition time; (b) 532 nm; 7 × 10²³ photons cm^–2 s^–1; 100 ms acquisition time. The spectra in (a) are averages of 10 baseline corrected spectra. Bands marked with an asterisk in (b) are due to the ethanol in the solvent.

Figure 6

(a) Nonresonant Raman spectra, (b) hyper-Raman spectra, and (c) resonant Raman spectra of 7′-apo-7′-(4-mercaptomethylphenyl)-β-carotene (upper spectra) and of the nonfunctionalized β-carotene (lower spectra) measured from solid samples. Excitation: (a) 1064 nm; peak photon flux density 3 × 10²⁷ photons cm^–2 s^–1; acquisition time 20 s; (b) 1064 nm; peak photon flux density 1 × 10²⁸ photons cm^–2 s^–1; acquisition time 600 s; (c) 532 nm; peak photon flux density 1 × 10²³ photons cm^–2 s^–1 (upper spectrum); 5 × 10²³ photons cm^–2 s^–1 (lower spectrum); 1 s acquisition time.