Optimization of a label‐free biosensor vertically characterized based on a periodic lattice of high aspect ratio SU‐8 nano‐pillars with a simplified 2D theoretical model

Abstract

We have recently demonstrated a biosensor based on a lattice of SU8 pillars on a 1 μm SiO2/Si wafer by measuring vertically reflectivity as a function of wavelength. The biodetection has been proven with the combination of Bovine Serum Albumin (BSA) protein and its antibody (antiBSA). A BSA layer is attached to the pillars; the biorecognition of antiBSA involves a shift in the reflectivity curve, related with the concentration of antiBSA. A detection limit in the order of 2 ng/ml is achieved for a rhombic lattice of pillars with a lattice parameter (a) of 800 nm, a height (h) of 420 nm and a diameter(d) of 200 nm. These results correlate with calculations using 3D‐finite difference time domain method. A 2D simplified model is proposed, consisting of a multilayer model where the pillars are turned into a 420 nm layer with an effective refractive index obtained by using Beam Propagation Method (BPM) algorithm. Results provided by this model are in good correlation with experimental data, reaching a reduction in time from one day to 15 minutes, giving a fast but accurate tool to optimize the design and maximizing sensitivity, and allows analyzing the influence of different variables (diameter, height and lattice parameter). Sensitivity is obtained for a variety of configurations, reaching a limit of detection under 1 ng/ml. Optimum design is not only chosen because of its sensitivity but also its feasibility, both from fabrication (limited by aspect ratio and proximity of the pillars) and fluidic point of view. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Efficient design and optimization of bio-photonic sensing cells (BICELLs) for label free biosensing

“…This model changes the strategy of optimization of previous works [10] in order to achieve a faster and more efficient methodology that does not imply using the complex Finite Difference Time Domain algorithm (FDTD) [11]. This analytical ID model is shown schematically in Fig.…”

“…Critical parameters in the fabrication of the cells are the diameter and the lattice parameter (or pitch) of the pillars. As a general rule, a higher compacting of pillars resulted on better values of LOD and sensitivity [10], but it makes also more complex the fabrication process. Finally the introduction of extra interferometric layers, for example of SÍO2 does not complicate the fabrication process, but can represent a gain in terms of several of the figures of merit analyzed.…”

“…Past studies suggested that the variation of any of the design parameters of the periodic lattice has an influence in the expected shift of interference, and thus in the biosensing performance of the device [10]. The goal of this work is define this design process, by identifying which are the most important parameters and state its influence in the final performance.…”

Caracterización y optimización de estructuras biofotónicas y técnicas avanzadas de interrogación para desarrollo de biosensores ópticos

“…El modelo de índices equivalentes se valida por comparación con datos experimentales propios de espectrometría obtenidos por nuestro grupo de investigación , [Casquel, 2011], . Una vez comprobada la validez práctica del modelo, éste se utiliza para simular cómo varía la sensibilidad de los biosensores con los diferentes parámetros que los definen: materiales, tipo de estructuras, dimensiones, etc.…”

“…El grupo de investigación de Óptica, Micro-Nano-Fotónica y Biofotónica utiliza como sistema de simulación el programa Rsoft®[Rsoft], basado en el algoritmo de propagación de haz (Beam Propagation Method, BPM) y el método de las diferencias finitas en el dominio del tiempo (Finite Difference Time Domain Method, FDTD) Esta última técnica es capaz de proporcionar soluciones en geometrías que donde el método de propagación de haz no puede aplicarse; como contrapartida, los tiempos de computación serán extremadamente largos [Scarmozzino, 2000]. Aun en el caso de utilizar el algoritmo BPM; y un modelo simplificado bidimensional de las estructuras basado en índices de refracción efectivos de la zona externa, la simulación de la respuesta óptica de una sola estructura puede durar decenas de minutos [Casquel, 2011]. En el artículo citado en la frase anterior, se describe cómo, utilizando este modelo, en vez de el modelo tridimensional FDTD, el tiempo de computación disminuye del orden de días al de minutos, obteniéndose además, una buena correlación entre el modelo bidimensional, el tridimensional y los datos experimentales que se aportan en el artículo.…”