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Fluorescent Tryptophan-Doped Silica Microparticles
Prepared Through a Reverse Microemulsion Method

Janine Marriah G. Dela Cruz1,2 and Christopher Jay T. Robidillo1*

1Department of Physical Sciences and Mathematics
College of Arts and Sciences, University of the Philippines Manila
2College of Medicine, University of the Philippines Manila

*corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

ABSTRACT
A reverse microemulsion method was utilized in synthesizing silica microparticles doped with tryptophan molecules as fluorophore. The fluorescent microparticles were formed inside reverse micelles dispersed in a cyclohexane continuous phase. The microparticles were produced within 24 hours and showed strong emission at a wavelength of 285 nm. The blue shift in their fluorescence maximum can be attributed to the caging and confinement effects of the silica network on the encapsulated tryptophan molecules. Successful encapsulation of tryptophan was confirmed through Fourier Transform Infrared Spectroscopy and Energy-Dispersive X-Ray Spectroscopy. Scanning Electron Microscopy and Dynamic Light Scattering Analysis revealed that the diameters of tryptophan-doped silica microparticles were in the range of 203 to 692 nm in the solid state, and in the range of 223 to 341 nm, with a narrow size distribution centered at 282 nm, in aqueous solution. Properties relevant to probe applications such as photostability and fluorophore leakage were also investigated. Tryptophan-doped silica microparticles were found to maintain their photostability even after six hours of continuous exposure to a 150 W halogen lamp and were observed to not undergo tryptophan leakage after three days of aqueous dispersion. This study has effectively extended dye encapsulation in silica to a biologically endogenous fluorescent amino acid, yielding fluorescent microparticles with desirable properties for fluorescent probes, namely, biocompatibility, photostability, non-leakage, monodispersity in solution, and fairly uniform sizes.

INTRODUCTION
Fluorescence spectroscopy is a luminescent technique that has gained considerable interest due to its ease of implementation, and high sensitivity and selectivity. Innovations in fluorescent analytical techniques have been applied in the development of sensors for probing chemical and biological processes (Basabe-Desmonts et al. 2006). These probes were successfully used in biological research, clinical diagnosis, and detection of various kinds of substances (Lian et al. 2004; Liaud et al. 2015; Liu et al. 2015; Hodáková et al. 2015; Qin et al. 2016). . . . read more

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