Abstract

Operando and precisely probing aqueous pH is fundamentally demanded, both in chemical and biological areas. Conventional pH probes, subjected to the larger size, are probably unfit for application in some extreme scenarios, such as a trace amount of samples. In this paper, we have further developed the pH sensor that leverages the microfiber Bragg grating with an ultra-compact size down to an order of magnitude of 10−14 m3. Using the electrostatic self-assembly layer-by-layer technique, the functional film consisting of sodium alginate, which harnesses a pH-dependent hygroscopicity, is immobilized on the fiber surface. Consequently, the alteration of aqueous pH could be quantitatively indicated by the wavelength shift of the grating resonance via the refractive index variation of the sensing film due to the water absorption or expulsion. The grating reflections involving fundamental mode and higher order mode exhibit the sensitivities of −72 pm/pH and −265 pm/pH, respectively. In addition, temperature compensation can be facilitated by the recording of the two reflections simultaneously. Furthermore, the modeling and simulation results predict the pivotal parameters of the configuration in sensitivity enhancement. The proposed proof-of-concept enriches the toolbox of pH sensor for catering to the need of detection in some extremely small spaces—for example, the living cells or the bio-tissues.

Highlights

  • A myriad of applications, such as chemistry analyzation, environmental protection, the food industry, drug processing and medical diagnostics, necessitate the evaluation of several vital chemical indicators

  • The hydrochloric acid (HCl) and NaOH were the preferred solvents that can minimize the potential disturbance caused by the refractive index variation of the solutions

  • As a promising photonic device that has conducted in bio/chemical sensing, microfiber

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Summary

Introduction

A myriad of applications, such as chemistry analyzation, environmental protection, the food industry, drug processing and medical diagnostics, necessitate the evaluation of several vital chemical indicators. Thereinto, the alteration of aqueous pH is capable of unraveling the chemical reaction and biological activity. Tremendous efforts, as a result, have been devoted to facilitating the real-time and in situ pH detection technique for meeting the requirements of the scenarios mentioned above. A pH probe possessing the superiorities of versatility, flexibility, and compactness is highly pursued. The fiber-optic pH sensor becomes a competitive candidate that precedes in slenderness, maturity and low-cost of fabrication, variability, high sensitivity, biological compatibility, electromagnetic immunity and capability of remote manipulation [1,2,3]. Various sensing strategies involving leveraging optical fibers have been involved in enabling the pH measurement

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