DOI: 10.15129/18d5b03c-d906-48a0-a68b-afae4ac55e74
title: Determination of the Orientation and Bonding of a Near Infrared Dye to Hollow Gold Nanospheres – A Chalcogen Tripod

description: Infrared surface enhanced Raman scattering (SERS) is an attractive option for the in situ detection of nanoprobes in biological samples due to the greater depth of penetration and reduced interference compared to SERS in the visible region. A key challenge is to understand the surface layer formed in suspension when a specific label is added to the SERS substrate in aqueous suspension.  SERS taken at different wavelengths, density functional theory (DFT), and surface-selective sum frequency generation vibrational spectroscopy (SFG-VS) were used to define the surface orientation and manner of attachment of a new class of infrared SERS labels with a thiopyrylium core and four pendant 2-selenophenyl rings. Hollow gold nanospheres are used as the substrate. Two distinct types of SERS spectra are obtained. With excitation close to resonance with both the near infrared electronic transition in the label (max 826 nm) and the plasmon resonance maximum (690 nm), surface enhanced resonance Raman scattering (SERRS) is obtained. SERRS indicates that the major axis of the core is near to perpendicular to the surface plane and SFG-VS gives a similar orientation with the major axis at an angle 72°-85° from the surface plane. New bands appear in SERS due to vibrations with significant displacements between the thiopyrylium core and the pendant selenophene rings. Analysis using calculated spectra with one or two rings rotated indicate that two rings on one end are rotated towards the metal surface to give an arrangement of two selenium and one sulphur atom directly facing the gold structure. The spectra together with a space filled model suggest that the molecule is strongly adsorbed to the surface through the selenium and sulphur atoms in an arrangement which will facilitate layer formation.

Figure 1 - Molecular structure of dye 1 with ring numbers (Se1 – Se4) and showing the torsion angles. 

Figure 2 - (a) Calculated and experimental electronic spectra for dye 1. (b) extinction spectrum for the HGN alone, with dye 1 added and with the labeled HGN aggregated; (c) electron density of the π (left) and π*  (right) orbitals which give the 826-nm band in (a) and (b). 

Figure 3- Normalized resonance Raman spectra for dye 1 at three excitation wavelengths (514, 633, and 785 nm).

Figure 4 - For dye 1, comparison of the 785-nm resonance spectrum with the corrected theoretical spectrum and displacement diagrams for the 1588 cm-1 vibration.

Figure 5 - SERRS of dye 1 obtained with 785-nm and 633-nm excitation compared to resonance spectra.

Figure 6 - SERS of dye 1 obtained with 1064- and 1280-nm excitation.

Figure 7 - SERS spectrum of dye 1 taken with 1280-nm excitation and compared to the theoretical calculation with the Se1 or Se3 ring or both rings rotated.

Figure 8 - Extreme positions of the vibrational displacements for the vibration at 1179 cm-1 (a) with the positions superimposed and for the vibration at 732 cm-1 (b) with each extreme shown separately.

Figure 9 - (a) The ssp, ppp, and sps SFG spectra of dye 1 on a polycrystalline gold film. (b)  PNA results for the 1600-cm-1 resonance showing a minimum at a polarization angle of -65.2°. (c) The molecular frame coordinate system used for orientation analysis of the pyrylium backbone and the relationship between the macroscopic laboratory frame and microscopic coordinate system (below).  (d) Simulation of the ssp to ppp SFG intensity ratio for the 1600-cm-1 symmetric stretch. 

Figure 10 - Space filled model of the dye on the surface showing the two selenophene rings rotated and the position of the thiopyrylium S atom.