Modeling and Development of a Biosensor Based on Optical Relaxation Measurements of Hybrid Nanoparticles

Abstract

We present a new approach for homogeneous real-time immunodiagnostics (denoted as "PlasMag") that can be directly carried out in sample solutions such as serum, thus promising to circumvent the need of sample preparation. It relies on highly sensitive plasmon-optical detection of the relaxation dynamics of magnetic nanoparticles immersed in the sample solution, which changes when target molecules bind to the surfaces of the nanoparticles due to the increase in their hydrodynamic radii. This method requires hybrid nanoparticles that combine both magnetic and optical anisotropic properties. Our model calculations show that core-shell nanorods with a cobalt core diameter of 6 nm, a cobalt core length of 80 nm, and a gold shell thickness of 5 nm are ideally suited as nanoprobes. On the one hand, the spectral position of the longitudinal plasmon resonance of such nanoprobes lies in the near-infrared, where the optical absorption in serum is minimal. On the other hand, the expected change in their relaxation properties on analyte binding is maximal for rotating magnetic fields as excitation in the lower kHz regime. In order to achieve high alignment ratios of the nanoprobes, the strength of the magnetic field should be around 5 mT. While realistic distributions of the nanoprobe properties result in a decrease of their mean optical extinction, the actual relaxation signal change on analyte binding is largely unaffected. These model calculations are supported by measurements on plain cobalt nanorod dispersions, which are the base component of the aspired core-shell nanoprobes currently under development.

Homogeneous Protein Analysis by Magnetic Core–Shell Nanorod Probes

“…Furthermore, the impact of the magnetic field amplitude and its rotational frequency on the obtained phase lag difference Δα is shown in the Supporting Figure S5 in a 3-dimensional plot. Noticeably, for each magnetic field amplitude there is an optimum field frequency at which Δα reaches a maximum, 12 but the maximum value of Δα increases with increasing field amplitude, which is attributed to the increasing alignment ratio of the nanoprobes 11 . The dependence of Δαmax on the magnetic field amplitude, however, is rather weak.…”

“…By correlating the momentary 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 vector of the applied magnetic field to the measured alignment of the nanoprobes, the phase lag angle of the nanoprobes relative to the applied field direction can be deduced, which is characteristic of the nanoprobe dynamics and increases on target protein binding (see Figure 1 caption for details). 11 The magnetic excitation mechanism characteristic to our platform enables application of frequency-filtering techniques to the detection signal, which efficiently suppresses interferences originating from complex media and allows us to specifically analyze processes at the nanoprobe surface only. For the experimental realization of the method, see Supporting Section 1.…”

Integrated optical waveguide and nanoparticle based label-free molecular biosensing concepts

“…It is based on optically observing the rotational dynamics of noble metal coated magnetic nanorods 27,28 . As biomarker proteins specifically bind to the antibody-functionalized nanorods, they significantly increase the hydrodynamic drag, thus leading to an increased phase lag of the orientation of the nanorods relative to the rotating magnetic field.…”

Chemical synthesis, structure and magnetic properties of Co nanorods decorated with Fe3O4 nanoparticles

“…In the last few years, a variety of inorganic heterostructured nanocomposites have been synthesized by the solution chemistry route [13,14], such as FePt-Co (or Ni, Fe 2 C, Au) nanoparticles [15,16], FePd-Fe 3 O 4 urchin-like nanocomposites [17], Au/Ag-Fe 3 O 4 dumbbell-like nanostructures [18], and metal-tipped semiconductor nanorods (NRs) [19,20], providing a new approach to develop diverse heterostructured nanocomposites with high performance as well as novel applications. More importantly, the one-dimensional (1D) heterostructures which contain a nanorod base decorated with a functional layer, cause great concern of the researchers [21,22], and these materials are ready for manufactured devices and ideal building blocks in varied applications [23][24][25][26].…”