Quantitative measurements of absorption spectra in scattering media by low-coherence spectroscopy

Low-coherence spectroscopy (LCS) is a spectroscopic method that allows for quantitative and localized assessment of absorption spectra by combining reflection spectroscopy with low-coherence interferometry. We describe absorption coefficient (µa) measurements by LCS in tissue simulating phantoms with varying scattering and absorbing properties. We used LCS in the 455–680 nm wavelength range with a spectral resolution of 8 nmto obtain µa spectra with ±0.5 mm−1 accuracy. We conclude that LCS is a promising technique for the in vivo determination of tissue chromophore concentrations

Angular scanning and variable wavelength surface plasmon resonance allowing free sensor surface selection for optimum material- and bio-sensing

A variable-wavelength Kretschmann configuration surface plasmon resonance (SPR) apparatus with angle scanning is presented. The setup provides the possibility of selecting the optimum wavelength with respect to the properties of the metal layer of the sensorchip, sample matrix, and biomolecular interaction of interest. Monitoring SPR curves over a wide angular range (39°) permits simultaneous determination of the total internal reflection angle (TIR), the resonance angle, and the intensity and width of the SPR dip, which are essential parameters for measuring binding events and achieving optimum sensitivity. The new apparatus was evaluated by recording full SPR curves at different wavelengths ranging from 600 to 890 nm using sensor surfaces of silver, gold and gold with deposited silicon oxide, aluminum oxide, titanium oxide and indium tin oxide which were exposed to air and an aqueous solution of sodium chloride. Clear wavelength dependencies of sensor-material resonance angles and SPR-dip widths were demonstrated. In order to investigate the capability of the system to probe molecular binding to different sensor surfaces, the layer-by-layer adsorption of charged polyelectrolytes was monitored in angular scanning mode at 600, 670, 785, and 890 nm. Although at longer wavelengths lower angular shifts were observed as result of layer deposition, the sharper dip, wider detection window and better signal-to-noise ratios at these wavelengths can be beneficial for binding studies. The applicability for biosensing was tested by immobilizing human serum albumin (HSA) on an aluminum-oxide-coated gold sensor using a new procedure and measuring the binding of anti-HSA antibodies at the optimal wavelength (890 nm) in angular-scanning and fixed-angle mode. The HSA biosensor showed negligible non-specific interaction and yielded almost ten times better sensitivity than obtained with a conventional gold-dextran-based sensor operated at 670/785 nm. Analysis of anti-HSA samples pre-incubated with different concentrations of HSA allowed measurement of the IC50 value. The reported data demonstrate the usefulness of the presented variable-wavelength angle-scanning SPR instrument, permitting continuous recording of full SPR curves in time at any selected wavelength in the 600–890 nm range using a sensor material of choice