A Nanoscale Optical Biosensor:  Real-Time Immunoassay in Physiological Buffer Enabled by Improved Nanoparticle Adhesion

Abstract

The shift in the extinction maximum, λmax, of the localized surface plasmon resonance (LSPR) spectrum of triangular Ag nanoparticles (∼90 nm wide and 50 nm high) is used to probe the interaction between a surface-confined antigen, biotin (B), and a solution-phase antibody, anti-biotin (AB). Exposure of biotin-functionalized Ag nanotriangles to 7 × 10-7 M < [AB] < 7 × 10-6 M caused a ∼38 nm red-shift in the LSPR λmax. The experimental normalized response of the LSPR λmax shift, (ΔR/ΔR max), versus [AB] was measured over the concentration range 7 × 10-10 M < [AB] < 7 × 10-6 M. Comparison of the experimental data with the theoretical normalized response for a 1:1 binding model yielded values for the saturation response, ΔR max = 38.0 nm, the surface-confined thermodynamic binding constant, K a,surf = 4.5 × 107 M-1, and the limit of detection (LOD) < 7 × 10-10 M. The experimental saturation response was interpreted in terms of a closest-packed structural model for the surface B−AB complex in which the long axis of AB, l AB = 15 nm, is oriented horizontally and the short axis, h AB = 4 nm is oriented vertically to the nanoparticle surface. This model yields a quantitative response for the saturation response, ΔR max = 40.6 nm, in good agreement with experiment, ΔR max = 38.0 nm. An atomic force microscopy (AFM) study supports this interpretation. In addition, major improvements in the LSPR nanobiosensor are reported. The LSPR nanobiosensor substrate was changed from glass to mica, and a surfactant, Triton X-100, was used in the nanosphere lithography fabrication procedure. These changes increased the adhesion of the Ag nanotriangles by a factor of 9 as determined by AFM normal force studies. The improved adhesion of Ag nanotriangles now enables the study of the B−AB immunoassay in a physiologically relevant fluid environment as well as in real-time. These results represent important new steps in the development of the LSPR nanosensor for applications in medical diagnostics, biomedical research, and environmental science.

Localized Surface Plasmon Resonance Nanosensor: A High-Resolution Distance-Dependence Study Using Atomic Layer Deposition

“…Figure 5A shows the QCM measurements recorded during 100 AB cycles of Al 2 O 3 ALD. The QCM signals have been converted to Al 2 O 3 thickness assuming a density of 2.6 g/cm 3 . 18 The Al 2 O 3 growth is extremely linear with a growth rate of 1.0 Å/cycle in excellent agreement with the VASE and SEM results.…”

Shift of localized surface plasmon resonance by Ar-ion irradiation of Ag–Au bimetallic films deposited on Al2O3 single crystals

“…In the past decades, metallic nanoparticles either sustained on surfaces or dispersed in dielectric matrices have been extensively studied because of their pronounced optical and electrical properties [1][2][3][4][5][6][7][8]. Most of the studies have focused on localized surface plasmon excitation, which dominates the photoabsorption spectra in the visible range.…”

Selective synthesis of hexagonal Ag nanoplates in a solution-phase chemical reduction process

“…Among the multipolar resonant modes, the in-plane dipole resonance peak is found to be very intense and sensitive to the particle size and aspect ratio, as well as to the surrounding medium, which allows the extinction wavelength to be tuned from the visible to the near-IR region [2,3]. The sensitivity of the in-plane resonance peak to changes in the surrounding medium has been utilized in biological and chemical sensing of, e.g., proteins [4], polypeptides [5], anti-biotin [6] or carbohydrates [7]. The surface plasmons of an Ag nanoplate can also enhance the evanescent waves of a separated object.…”