Investigations on the Q and CT Bands of Cytochrome c Submonolayer Adsorbed on an Alumina Surface Using Broadband Spectroscopy with Single-Mode Integrated Optical Waveguides

Abstract

In this work, we report experimental results on the molar absorptivity of cytochrome c adsorbed at different submonolayer levels onto an aluminum oxide waveguide surface; our data show a clear dependence of the protein optical properties on its surface density. The measurements were performed using the broadband, single-mode, integrated optical waveguide spectroscopic technique, which is an extremely sensitive tool able to reach submonolayer levels of detection required for this type of studies. This investigation focuses on the molar absorptivity at the Q-band (centered at 525 nm) and, for the first time to our knowledge, the weak charge transfer (CT) band (centered at 695 nm) of surface-adsorbed cyt c. Polarized light in the spectral region from 450 to 775 nm was all-coupled into an alumina thin film, which functioned as a single-mode planar optical waveguide. The alumina thin-film waveguide used for this work had a thickness of 180 nm and was deposited on a glass substrate by the atomic layer deposition process. The protein submonolayer was formed on the alumina waveguide surface through electrostatic adsorption from an aqueous buffer solution at neutral pH. The optical properties of the surface-adsorbed cyt c were investigated for bulk protein concentrations ranging from 5 nM to 8200 nM in the aqueous buffer solution. For a protein surface density of 2.3 pmol/cm 2 , the molar absorptivity measured at the charge transfer band was 335 M −1 cm −1 , and for a surface density of 15 pmol/cm 2 was 720 M −1 cm −1 , which is much closer to the value of cyt c dissolved in an aqueous neutral buffer (830 M −1 cm −1 ). The modification of the protein molar absorptivity and its dependence on the surface density can most likely be attributed to conformational changes of the surface-adsorbed species.

Low-loss optical waveguides for the near ultra-violet and visible spectral regions with Al2O3 thin films from atomic layer deposition

“…Strong out-coupling qualitatively and visually corroborates the previously discussed quantitative measurements of low propagation loss in our ALD-deposited alumina films. These experimental results confirm the ability of the ALD process to produce low loss optical waveguides well inside the UV region; such ability is expected to become extremely useful for developing integrated optic devices in the near-UV and visible spectral regions for a variety of applications including sensing of many relevant biological materials (e.g., see [25] for specific applications).…”

“…Such experimental results will certainly enable the fabrication of several integrated optic devices in the short wavelength spectral region for a variety of applications. In particular, our group has already started exploiting the high quality of ALD-deposited alumina waveguides and we have incorporated those films as part of a single-mode integrated optical waveguide platform for spectroscopic studies of surface-adsorbed protein sub-monolayers [25]. …”

A displacement principle for mercury detection by optical waveguide and surface enhanced Raman spectroscopy

“…100 nm and was absorbed by the species on OWG surface [24,25]. The multiple total reflections of I 0 yielded the high sensitivity, and the exclusion of light-absorbing species in the bulk of the solution from contributing to the extinction spectrum produced the surface specificity of the OWG spectroscopy.…”

Effects of sodium chloride on the properties of chlorophyll <italic>a</italic> submonolayer adsorbed onto hydrophobic and hydrophilic surfaces using broadband spectroscopy with single-mode integrated optical waveguides

“…2) for the data acquisition reported in this study was developed in our laboratories and reported in detail elsewhere. 31,32,35 The setup used is composed of a broadband light source, a single-mode planar optical waveguide with integrated grating couplers, a flowcell, a monochromator, and a charge coupled device (CCD) array detector. The light source used was a tungsten-halogen lamp (Philips, FocusLine) with 4.5 V and 2.55 A.…”