Optical properties of a new inorganic liquid crystal Jessica C. Ramella-Roman, Otto C. Wilson Department of Biomedical Engineering The Catholic University of America, Washington DC, 20064 Abstract - We measured the optical properties of a new inorganic liquid crystal material. By changing the synthesization process AlFe colloidal rods approximately 200-400 nm in length and ellipsoidal platelets 2000-5000 nm in diameter can be created. In this paper we show that the optical properties of this new material change with its exposure to an external magnetic field making it particularly suitable for medical diagnostic applications. I. INTRODUCTION Inorganic liquid crystals are rare [1,2], unlike their organic counterpart. Their controllable self-assembly makes them a promising building block in the construction of smart materials suited for medical devices. A new inorganic liquid crystal phase was synthesized by hydrothermal aging of Al/Fe gels resulting in a distribution of colloidal rod shaped particles of 200-400 nm length and platelets 2000- 5000 nm in diameter. The procedure to generate rods and platelets is described elsewhere [3], and it is based on previous methods by Souza Santos et al. [4]. The resulting nanoparticles have interesting mechanical and optical properties when positioned in a magnetic field. Depending on the pH reaction Ω solutions of different color may be achieved [3]. Previous tests have shown that the color change is based on the relative size of AlFe particles and their orientation. The color of the measured colloidal suspensions of nanoparticles changed from red to brown depending on their proximity to a permanent magnet. The development of the nanoparticles is currently being explored in relation to the study of artificial bone- grafts. By using the nanoparticles ability to induce shear stress on the graft by means of an external magnetic field, graft growth and repair could potentially be facilitated. In this context we are developing optical methods for monitoring the nanoparticle orientation. In this paper we illustrate a set of measurements aimed at quantifying the wavelength dependent reflectance of the AlFe particles under the influence of a magnetic field. II. MATERIAL AND METHODS The reflectance from a solution of AlFe particles was measured with the aid of the goniometric system shown in Fig.1. A 5 cm diameter and 2 cm thickness NdFeB magnet (14,700 Gauss, Applied Magnets, TX) was encased in a plastic support and positioned on the rotating arm of a computer controlled goniometer (Newport, Irvine, CA). A 1 cm diameter cylindrical cuvette was suspended in the center of the goniometer at less than 0.1 cm from the magnet. The cuvette contained 5 ml of an AlFe colloidal solution. The magnet could rotate around the cuvette in steps of 10 degrees maintaining the same cuvette-magnet distance, the total travel was 0 o to 90 o . A fiber-based spectrophotometer (Ocean Optics, FL) was used to measure the reflected light spectrum for each magnet position. A 600µm core multi-mode fiber optic was positioned in front of the cuvette and connected to the spectrophotometer and was used as the collection fiber. A second fiber of the equal size was positioned next to the collection fiber and was connected to a tungsten-halogen white light source (Ocean Optics, FL). The emission fiber generated a spot size of about 2 mm in diameter on the cuvette. Fig. 1. Experimental layout magnet- 0 position magnet 90 position Al/Fe cuvette Spectrophotometer White Light Source 77 TuB2.3 11.30 - 11.45 978-1-4244-1926-5/08/$25.00 © 2008 IEEE