APTES modification of optical fibers: Dip coating and silanization approaches

This study compares two (3-aminopropyl) triethoxysilane (APTES)-based surface functionalization techniques—silanization and dip coating—for modifying silica optical fibers intended for sensing applications. Here, six concentrations of APTES (0.5%–3.0%) were investigated, and the resulting fiber properties were evaluated using gravimetric methods, adsorption isotherm plots, Fourier-transform infrared (FTIR) spectroscopy, and field-emission scanning electron microscopy (FESEM)–energy-dispersive X-ray spectroscopy (EDX). Gravimetric results showed an increase in APTES mass accumulation for both methods. Dip coating consistently produced higher surface loading, with a maximum mean mass of 2.33 mg compared with 2.00 mg for silanization. The process adhered to a Freundlich isotherm ( R 2 > 0.98), with the calculated standard Gibbs free energy ranging between −8.30 and −8.67 kJ mol −1 , indicating spontaneous physisorption. Attenuated total reflectance–FTIR spectra confirmed successful APTES functionalization through the presence of characteristic C–H and N–H vibrational bands. FESEM analysis revealed that silanization produced stable, uniform thin films, whereas dip coating formed thicker, irregular multilayer structures with surface aggregation. EDX analysis further supported these observations by indicating a higher nitrogen content on the fiber surface for silanized samples (3.84at%) compared with the dip-coated samples (3.32at%). These findings demonstrate that dip coating enhances surface mass loading, whereas silanization produces more uniform and chemically stable functional layers, providing practical guidance for the design of optical fiber biosensors. • APTES modification of optical fibers via silanization and dip coating • Dip coating yields higher mass loading via multilayer adsorption • Silanization forms uniform, stable films with exposed amine groups