Dual fluorescent hollow silica nanofibers for in situ pH monitoring using an optical fiber

Abstract

This study reports a sensitive and robust pH sensor based on dual fluorescent doped hollow silica nanofibers (hSNFs) for in situ and real-time pH monitoring. Fluorescein isothiocyanate (FITC) and tris(2,2'-bipyridyl)dichlororuthenium(ii) hexahydrate (Ru(BPY)₃) were chosen as a pH sensitive dye and reference dye, respectively. hSNFs were synthesized using a two-step method in a reverse micelle system and were shown to have an average length of 6.20 μm and average diameter of 410 nm. The peak intensity ratio of FITC/Ru(BPY)₃ was used to calibrate to solution pH changes. An optical-fiber-based fluorescence detection system was developed that enabled feasible and highly efficient near-field fluorescence detection. The developed system enables fully automated fluorescence detection, where components including the light source, detector, and data acquisition unit are all controlled by a computer. The results show that the developed pH sensor works in a linear range of pH 4.0-9.0 with a fast response time of less than 10 s and minimal sample volume of 50 μL, and can be stored under dark conditions for one month without failure. In addition, the as-prepared hSNF-based pH sensors also have excellent long-term durability. Experimental results from ratiometric sensing confirm the high feasibility, accuracy, stability and simplicity of the dual fluorescent hSNF sensors for the detection of pH in real samples.

Fig. 1. Schematic of the dual fluorescent hSNF pH sensor and the optical characterization setup. The left upper inset is a microscope image of the tilted optical fiber tip positioned above the hSNFs.

Fig. 2. Schematics of the hSNF synthesis, surface coating and deposition process. (a) Synthesis of an hSNF containing Ru(BPY)₃ in the water-in-oil microemulsion. (b) Reaction of FITC and APTES to form a conjugate molecule. (c) Coating process of the FITC–APTES conjugate on the surface of an SNF. (d) Plasma treatment of the polystyrene material. (e) Deposition of the dual fluorescent hSNFs onto the plasma treated Petri dish.

Fig. 3. Characterization of hSNFs. (a–c) SEM images of hSNFs. hSNFs (b) before and (c) after FITC coating. (d–f) TEM images of hSNFs at different scales. (g and h) Fluorescence images of FITC and Ru(BPY)₃. (i) Bright field images of hSNFs on the polystyrene substrate.

Fig. 4. EDS characterization of dual fluorescent hSNFs. (a) The EDS energy spectrum of dual fluorescent hSNFs and (b) its enlarged view showing the peaks of elements S and Ru. (c) Field of view of the EDS map. EDS elemental mapping of (d) Si, (e) S and (f) Ru.

Fig. 5. Emission spectra of fluorophores and the dual fluorescent hSNFs. (a) Emission spectra of FITC dissolved in water and FITC coated hSNFs dissolved in water. (b) Emission spectra of Ru(BPY)₃ dissolved in water and Ru(BPY)₃ coated hSNFs dissolved in water. (c) Emission spectrum of hSNFs coated with both FITC and Ru(BPY)₃.

Fig. 6. Characterization of the pH response of the dual fluorescent hSNFs. (a) The optical spectra of the dual fluorescent hSNFs collected at different locations with error bars (the shaded area). The inserted figure indicates the I_FITC/I_Ru(BPY)3 ratio at each location. (b) Optical images of the optical fiber collecting signals at different distances. (c) Linear fitting of the I_FITC/I_Ru(BPY)3 ratio against different pH values. (d) The optical spectra of the dual fluorescent hSNFs measured in pH 5.0 buffer solution in different cycles. (e) The optical spectra of the dual fluorescent hSNFs measured in pH 8.0 buffer solution in different cycles. (f) The plot of the I_FITC/I_Ru(BPY)3 ratio at different cycles calculated from the spectra in (d) and (e). (g) The optical spectra of the dual fluorescent hSNFs measured in the pH 7 buffer solution over 5 hours. (h) The plot of the I_FITC/I_Ru(BPY)3 ratio at different elapsed times. The dashed line represents the value of the I_FITC/I_Ru(BPY)3 ratio at pH 7.0 obtained from the linear fitting.

Fig. 7. Real sample assay. The pH measurement results of tap water (a), meltwater (b) and hiPSC-CMs cell culture media (c) samples from the commercial pH meter and dual fluorescent hSNF pH sensor.