Fluid Imbibition-Coulped Laser Interferometry As a Tool for Optical Biosensing with Anodic Aluminum Oxide
Authors: Ferré-Borrull Josep , Eckstein Chris , Xifré-Pérez Elisabet , Marsal Lluis F 
Published: February 2020

Optofluidics is a technology based on the fusion between microfluidics and optics. It is related to a variety of phenomena in which an interaction between light and fluids is found. It can be applied to the control of fluid properties by light, to the design of variable geometry optical elements by means of the adaptability of the fluid or by using optics to interrogate fluid properties. On the other hand, anodic aluminum oxide (AAO) is a porous material obtained by the electrochemical anodization of aluminum with a growing interest in nanotechnology because of its cost-effective production process and its promising variety of applications[1]. Recently, we have introduced an optofluidic technique called fluid imbibition-coupled laser interferometry (FICLI), based on the study by optical means of the liquid infiltration dynamics into AAO. We demonstrated its high accuracy for the characterization of the material[2,3]. In this work, we explore the possibilities of using such a sensitive technique as a sensor of biological species.
Fluid imbibition-coupled laser interferometry consists of the measurement of the capillary filling dynamics of a fluid into the AAO nanopores. Such measurement is performed by monitoring light intensity interferometric fluctuations of a laser beam in real-time as the liquid infiltrates the pores. An illustration of the measurement system can be seen in the figure. AAO membranes with 50 µm-long pores are used (see SEM pictures in the figure) and the laser beam is reflected at its top and bottom interfaces creating a time-resolved intereference. Light intensity fluctuations are measured by a photodetector and a high-speed data acquisition system, and are analyzed on the basis of mathematical models of the filling kinetics. This technique has shown enough accuracy to demonstrate that AAO nanopores are actually conical with an angle as small as 0.007º.
The processing of the time-resolved interferograms, as the one shown in the figure, can be done in several ways. The simplest method consists of measuring the filling time of the pores from each side of the membrane and combine these measurements with the information about the fluid and the surface properties (viscosity, surface tension, contact angle) to obtain the values of the radius at each pore end. A more elaborate method permits the determination of the pore profile by the inversion of the filling dynamics integral equations provided accurate measurement of the time-resolved interferograms for the filling from each end of the pores are obtained. In this work we introduce an alternative method based on the discretization of the filling dynamics equations, what permits to obtain a direct and accurate determination of the average radius of the nanopores.
In order to evaluate the accuracy of this alternative method we applied different modifications to the pore geometry and we measured the pore radius modification by the different methods. Pore radius increase was achieved by applying controlled wet etching to the as-produced AOO membranes with increasing time. On the other hand, pore radius decrease was achieved by depositing successive bilayers of polyelectrolytes. Results show that the proposed technique is accurate enough to sense pore radius variations as small as 4 nm, in the range of the size of many biological macromoleclues such as proteins, antibodies or aptamers.
With this ability in mind, we have studied the application of the technique to sense such biological species. In this communication we will report our results on the measurement of the changes in the filling dynamics of AAO membranes after the functionalization of their surface and the attachment of the different molecules: i) proteins like bovine serum albumin (BSA), ii) the successive attachment of the IgG-aIgG antibody-antigen pair with a view to the application of the technique as an immunosensor and iii) the attachment of an aptamer and the detection of changes in their conformation.
References:
[1] Ferre-Borrull, J., Pallares, J., Macias, G., Marsal, L.F., "Nanostructural Engineering of Nanoporous Anodic Alumina for Biosensing Applications", Materials 7, 5225, 2014.
[2] Urteaga, R., Acquaroli, L.N., Koropecki, R.R., Santos, A., Alba, M., Pallares, J., Marsal, L.F., Berli, C.L.A., "Optofluidic Characterization of Nanoporous Membranes", Langmuir 29, 2784, 2013.
[3] Eckstein, C., Xifre-Perez, E., Porta-i-Batalla, M., Ferre-Borrull, J., Marsal, L. F., "Optical Monitoring of the Capillary Filling Dynamics Variation in Nanoporous Anodic Alumina toward Sensing Applications", Langmuir 32, 10467, 2016.
Acknowledgments:
This work was supported in part by the Spanish Ministry of Economy and competitiveness TEC2015-71324-R (MINECO/FEDER), the Catalan authority AGAUR 2014SGR1344, and ICREA under the ICREA Academia Award.

Figure 1