Using picosecond excitation at 1064 nm, surface-enhanced hyper-Raman scattering (SEHRS) spectra of the nucleobases adenine, guanine, cytosine, thymine, and uracil with two different types of silver nanoparticles were obtained. Comparing the SEHRS spectra with SERS data from the identical samples excited at 532 nm and with known infrared spectra, the major bands in the spectra are assigned. Due to the different selection rules for the one- and two-photon excited Raman scattering, we observe strong variation in relative signal strengths of many molecular vibrations obtained in SEHRS and SERS spectra. The two-photon excited spectra of the nucleobases are found to be very sensitive with respect to molecule–nanoparticle interactions. Using both the SEHRS and SERS data, a comprehensive vibrational characterization of the interaction of nucleobases with silver nanostructures can be achieved.

In this work, we report nonresonant surface-enhanced hyper-Raman (SEHRS) spectra of the amino acids tryptophan, histidine, phenylalanine, and tyrosine using silver nanoparticles. The spectra are obtained at an excitation wavelength of 1064 nm and compared to the corresponding surface-enhanced Raman scattering (SERS) spectra measured at 532 nm excitation. The majority of the bands in the SEHRS spectra are assigned. Important hallmarks of the spectra include strongly diminished or absent bands from the ring breathing modes. SEHRS and SERS spectra obtained from histidine and tyrosine indicate changes at slightly varied amino acid concentration. Small changes in the SEHRS spectra were more pronounced than variation in the corresponding SERS data, supporting the high sensitivity of the SEHRS spectra with respect to structural changes due to small variations in surface environment. The possibility to measure nonresonant SEHRS spectra of amino acids in solution and the complementary information obtained from the spectra demonstrates the potential of this method for future investigations of proteins and more complicated biological structures and their interaction with nanostructures.

Surface enhanced hyper Raman scattering (SEHRS) and its applicationsSurface enhanced hyper Raman scattering (SEHRS), spontaneous, two-photon excited, plasmon-enhanced Raman scattering, provides a wealth of vibrational information that can be useful in many directions of spectroscopy. As featured in:See Being regarded as a non-linear analogue of surface enhanced Raman scattering (SERS), SEHRS shares most of its properties, but also has additional characteristics. They include complementary spectroscopic information resulting from different selection rules and a stronger enhancement due to the non-linearity in excitation. In practical spectroscopy, this can translate to advantages, which include a high selectivity when probing molecule-surface interactions, the possibility of probing molecules at low concentrations due to the strong enhancement, and the advantages that come with excitation in the near-infrared. In this review, we give examples of the wealth of vibrational spectroscopic information that can be obtained by SEHRS and discuss work that has contributed to understanding the effect and that therefore provides directions for SEHRS spectroscopy. Future applications could range from biophotonics to materials research.

Surface-enhanced hyper-Raman scattering (SEHRS) is very useful for the vibrational characterization of organic and biological molecules and their interaction with noble metal nanostructures. Many potential applications should ideally make use of gold nanostructures in order to enhance both the excitation and the weak hyper-Raman light, rather than silver nanostructures. Here, we report high SEHRS enhancement from spherical gold nanoparticles with different particle diameters ranging from 30 to 70 nm and from gold nanorods. SEHRS data of the two molecules crystal violet and rhodamine 6G obtained at an excitation wavelength of 1064 nm, absorbance spectra, and finite-difference time domain simulations of the electromagnetic field enhancement provide evidence that the SEHRS enhancement relies on the formation of nanoaggregates, with higher SEHRS signals yielded with increasing size of the nanoparticles in the aggregates. Gold nanorods and their aggregates are shown to provide optical properties that are specifically suited to support enhancement of SEHRS. The reported results suggest that plasmon resonances at the excitation wavelength, as well as enhancement due to the lightning rod effect, can contribute significantly to the total SEHRS enhancement. From the different concentration dependence of the signals of the different molecules as well as from comparison with salt-induced aggregation, it is concluded that the specific analyte-induced aggregation determines the specific gold nanoaggregates’ geometry, arrangement, and interparticle distances. Understanding the influence of the nanoaggregate properties therefore is crucial for exploiting gold SEHRS nanosensors in future applications.

We report the two-photon excited nonresonant surface-enhanced hyper Raman scattering (SEHRS) spectra of six aromatic thiol molecules during their interaction with gold and silver nanostructures. SEHRS spectra were obtained from thiophenol, benzyl mercaptan, and phenylethyl mercaptan and from the three isomers 2-aminothiophenol (2-ATP), 3-aminothiophenol (3-ATP), and 4-aminothiophenol (4-ATP). All SEHRS spectra were excited off-resonance at a wavelength of 1064 nm and compared to surface-enhanced Raman scattering (SERS) spectra excited at 785 nm or at 633 nm. The SEHRS spectra show a different interaction of thiophenol, benzyl mercaptan, and phenylethyl mercaptan with silver and gold nanostructures. Density functional theory calculations were used to support band assignments, in particular, for the unknown SERS spectrum of 3-ATP, and identify a band of phenylethyl mercaptan as a vibrational mode unique to the SEHRS spectrum and very weak in the Raman and infrared spectra. 2-ATP, 3-ATP, and 4-ATP show a different interaction with gold nanostructures that was found to depend on pH. Bands in the SEHRS spectrum of 2-ATP could be assigned to 2,2′-dimercaptoazobenzene, suggested to be obtained in a plasmon-assisted reaction that occurred during the SEHRS experiment. The results provide the basis for a better characterization of organic thiols at surfaces in a variety of fields, including surface functionalization and plasmonic catalysis.