Development and Investigation of a Dual-Pad In-Channel Referencing Surface Plasmon Resonance Sensor

Herein, we describe the construction of a novel dual-pad referencing surface plasmon resonance (SPR) sensor utilizing electrolytic grafting of diazonium salts to individually functionalize two gold pads positioned in a single fluidic channel. Using a dove prism, a simple single axis optical train may be employed without compromising the analytical performance. Once functionalized, one pad is used as the analytical sensing pad for detection of molecular interactions while the other serves as the reference pad, compensating for background refractive index fluctuations. The reference pad effectively compensates bulk refractive index changes and temperature variations as well as other nonspecific effects. The sensor was applied to calibration of a pH-responsive polymer layer in the presence of bulk refractive index and temperature variations. Monitoring selective attachment of a protein is also demonstrated. To our knowledge, this is the first implementation of in-channel referencing SPR sensor utilizing diazonium salt-based surface chemistry

Adsorbate–Metal Bond Effect on Empirical Determination of Surface Plasmon Penetration Depth

The penetration depth of surface plasmons is commonly determined empirically from the observed response for adsorbate loading on gold surface plasmon resonance (SPR) substrates. However, changes in the SPR spectrum may originate from both changes in the effective refractive index near the metal surface and changes in the metal permittivity following covalent binding of the adsorbate layer. Herein, the significance of incorporating an additional adsorbate–metal bonding effect in the calculation is demonstrated in theory and in practice. The bonding effect is determined from the nonzero intercept of a SPR shift versus adsorbate thickness calibration and incorporated into the calculation of penetration depth at various excitation wavelengths. Determinations of plasmon penetration depth with and without the bonding response for alkanethiolate–gold are compared and are shown to be significantly different for a thiol monolayer adsorbate system. Additionally, plasmon penetration depth evaluated with bonding effect compensation shows greater consistency over different adsorbate thicknesses and better agreement with theory derived from Maxwell’s equation, particularly for adsorbate thicknesses that are much smaller (<5%) than the plasmon penetration depth. The method is also extended to a more practically applicable polyelectrolyte multilayer adsorbate system

DC Magnetron Sputtered Polyaniline-HCl Thin Films for Chemical Sensing Applications

Thin films of conducting polymers exhibit unique chemical and physical properties that render them integral parts in microelectronics, energy storage devices, and chemical sensors. Overall, polyaniline (PAni) doped in acidic media has shown metal-like electronic conductivity, though exact physical and chemical properties are dependent on the polymer structure and dopant type. Difficulties arising from poor processability render production of doped PAni thin films particularly challenging. In this contribution, DC magnetron sputtering, a physical vapor deposition technique, is applied to the preparation of conductive thin films of PAni doped with hydrochloric acid (PAni-HCl) in an effort to circumvent issues associated with conventional thin film preparation methods. Samples manufactured by the sputtering method are analyzed along with samples prepared by conventional drop-casting. Physical characterization (atomic force microscopy, AFM) confirm the presence of PAni-HCl and show that films exhibit a reduced roughness and potentially pinhole-free coverage of the substrate. Spectroscopic evidence (UV–vis, FT-IR, and X-ray photoelectron spectroscopy (XPS)) suggests that structural changes and loss of conductivity, not uncommon during PAni processing, does occur during the preparation process. Finally, the applicability of sputtered films to gas-phase sensing of NH₃ was investigated with surface plasmon resonance (SPR) spectroscopy and compared to previous contributions. In summary, sputtered PAni-HCl films exhibit quantifiable, reversible behavior upon exposure to NH₃ with a calculated LOD (by method) approaching 0.4 ppm NH₃ in dry air