A Microfluidic Long-Period Fiber Grating Sensor Platform for Chloride Ion Concentration Measurement

Jian-Neng Wang

doi:10.3390/s110908550

Abstract

Optical fiber sensors based on waveguide technology are promising and attractive in chemical, biotechnological, agronomy, and civil engineering applications. A microfluidic system equipped with a long-period fiber grating (LPFG) capable of measuring chloride ion concentrations of several sample materials is presented. The LPFG-based microfluidic platform was shown to be effective in sensing very small quantities of samples and its transmitted light signal could easily be used as a measurand. The investigated sample materials included reverse osmosis (RO) water, tap water, dilute aqueous sample of sea sand soaked in RO water, aqueous sample of sea sand soaked in RO water, dilute seawater, and seawater. By employing additionally a chloride ion-selective electrode sensor for the calibration of chloride-ion concentration, a useful correlation (R2 = 0.975) was found between the separately-measured chloride concentration and the light intensity transmitted through the LPFG at a wavelength of 1,550 nm. Experimental results show that the sensitivity of the LPFG sensor by light intensity interrogation was determined to be 5.0 × 10−6 mW/mg/L for chloride ion concentrations below 2,400 mg/L. The results obtained from the analysis of data variations in time-series measurements for all sample materials show that standard deviations of output power were relatively small and found in the range of 7.413 × 10−5−2.769 × 10−3 mW. In addition, a fairly small coefficients of variations were also obtained, which were in the range of 0.03%–1.29% and decreased with the decrease of chloride ion concentrations of sample materials. Moreover, the analysis of stability performance of the LPFG sensor indicated that the random walk coefficient decreased with the increase of the chloride ion concentration, illustrating that measurement stability using the microfluidic platform was capable of measuring transmitted optical power with accuracy in the range of −0.8569 mW/ to −0.5169 mW/ . Furthermore, the bias stability was determined to be in the range of less than 6.134 × 10−8 mW/h with 600 s time cluster to less than 1.412 × 10−6 mW/h with 600 s time cluster. Thus, the proposed LPFG-based microfluidic platform has the potential for civil, chemical, biological, and biochemical sensing with aqueous solutions. The compact (3.5 × 4.2 cm), low-cost, real-time, small-volume (∼70 μL), low-noise, and high-sensitive chloride ion sensing system reported here could hopefully benefit the development and applications in the field of chemical, biotechnical, soil and geotechnical, and civil engineering.

Highlights

In recent years, the technology of fiber optic sensors has been applied to the field of structural monitoring, infrastructure assessment, and some industrialized sectors [1,2,3,4]
We focused on the use of long-period fiber grating (LPFG)-based microfluidic chip as chloride ion concentration sensor and the light intensity of the LPFG in the output power was used to quantify the chloride ion concentrations of different kinds of waters, aqueous samples of sea sand soaked in reverse osmosis (RO) water, and seawater
The sample numbers (No.) and their corresponding material were as follows: No 1: air, No 2: 50-mL RO water, No 3: 50-mL tap water, No 4: 50-mL dilute aqueous sample of sea sand soaked in RO water, sampling from 1,000-g oven-dry sea sand immersed in 1,600-mL RO water; No 5: 50-mL

Summary

Introduction

The technology of fiber optic sensors has been applied to the field of structural monitoring, infrastructure assessment, and some industrialized sectors [1,2,3,4]. As has happened with other types of ion-concentration optical sensors [7,8,9,10,11], only a limited number of highly selective optical chloride sensors have been reported [6,12,13,14,15,16]. The most commonly reported techniques, in conjunction with an optical fiber, are based on the development of sensitive sensing materials as matrix for entrapment of optical transducers that can provide the function of selective exaction and chemical recognition of the analyte, a few examples such as fluorescence quenching of a fluorophore immobilized on a polymeric matix [8], Na+ ions adsorption on sol-gen porous film [11], and ion-selective membranes deposited on dielectric waveguides [9,10]. Only a few number of selective ionophores for chloride exist, such as indium(III) metalloporphyrins [17], alkyltin(IV) derivatives [18], a hydrogen-bonding ionophore [19], and a [14] mercuracarborand-3 (MC-3) ionophore [12,13]

Methods

Results

Conclusion