**3. Experimental**

#### *3.1. Reagents*

*3.1. Reagents*  Poly (vinyl chloride) (PVC) powder and rhodamine B (RB) from Wako Pure Chemical Industries,

Poly (vinyl chloride) (PVC) powder and rhodamine B (RB) from Wako Pure Chemical Industries, Ltd. (Osaka, Japan) were used. 2-nitrophenyl octyl ether (NPOE), tetrakis (4-fluorophenyl) borate, sodium salt, and dihydrate (NaTPBF) were purchased from Dojindo Laboratories (Kumamoto, Japan). Sep-Pak Plus CN, Sep-Pak Plus C18, Sep-Pak Plus C8, OASIS HLB 1cc, and Sep-Pak Plus PS 2 purchased from Waters (Milford, MA, United States) were used as the solid-phase extraction (SPE) columns for the removal of PHMB. All other chemicals of reagent grade were used as received. The PHMB concentration (M = mol dmƺ3) indicates the number of moles of ionic groups/volume (liter) of Ltd. (Osaka, Japan) were used. 2-nitrophenyl octyl ether (NPOE), tetrakis (4-fluorophenyl) borate, sodium salt, and dihydrate (NaTPBF) were purchased from Dojindo Laboratories (Kumamoto, Japan). Sep-Pak Plus CN, Sep-Pak Plus C18, Sep-Pak Plus C8, OASIS HLB 1cc, and Sep-Pak Plus PS 2 purchased from Waters (Milford, MA, United States) were used as the solid-phase extraction (SPE) columns for the removal of PHMB. All other chemicals of reagen<sup>t</sup> grade were used as received. The PHMB concentration (M = mol dm−3) indicates the number of moles of ionic groups/volume (liter) of polymer solution.

polymer solution.

*3.2. Preparation of Optode Film Containing L-RB*

#### *3.2. Preparation of Optode Film Containing L-RB*

An optode film containing L-RB was prepared as follows. The fabrication method is similar to our previous paper [14–16]. Take NPOE (5.0 g) and 24.0 mL of a mixed aqueous solution containing 75.0 μM RB and 1.0 M NaOH in a plastic tube, stir for 24 h at 55 ◦C, and extract L-RB into the NPOE phase. The NPOE phase (4.0 g) centrifuged from the aqueous phase (10 min) and PVC powder (0.80 g) were solubilized in 20 mL of tetrahydrofuran (THF). By means of immersing a quartz glass plate (length: 3.5 cm, width: 1.0 cm, thickness: 1 mm) in the THF solution for 5 s, an L-RB film for the measurement of fluorescence was fabricated. In the event of measurement of absorbance of the L-RB film, a glass plate (length of 3.5 cm, a width of 1.0 cm and thickness of 1 mm) was used in place of the quartz glass plate. The L-RB film was dried at room temperature for more than 2 h and then conditioned with CH3COOH-CH3COONa buffer solution (pH 4.0) for 24 h before starting an experiment. The fluorescence intensity of the L-RB film was measured for 10 min at λex = 561 nm and λem = 584 nm on a spectrofluorimeter (JASCO FP-750, JASCO Corporation, Tokyo, Japan). The measurement was performed by placing the L-RB film on the diagonal of a 1 cm square quartz cuvette (height 4.5 cm, length 1.25 cm, width 1.25 cm) containing a sample solution (3.0 mL). The thickness of the prepared L-RB film was about 0.2 mm. In the case of measurement of absorbance of the L-RB film, glass plates coated with L-RB films were fixed in disposable cuvettes (4.5 cm high, 1.25 cm long, and 1.25 cm wide) filled with 2.0 mL of PHMB solution (pH 4.0 adjusted with a 0.1M CH3COOH/CH3COONa buffer solution). The absorption spectrum of the L-RB films was measured with a spectrophotometer (JASCO, V-530-iRM, JASCO Corporation, Tokyo, Japan). All spectroscopic measurements were performed in batch mode. The L-RB film was used as a disposable sensor for one-shot measurement.

#### *3.3. Evaluation of Removal of PHMB by Using Several Solid-Phase Extraction Columns with a Flow Injection Analysis (FIA)*

The FIA system and its experimental conditions for evaluation of the removal of PHMB in commercially available CLDs using several solid-phase extraction (SPE) columns are the same as that for the measurement of PHMB in the previous paper [11]. A PHMB sample solution (140 μL) with and without pretreatment using the SPE columns was injected into the FIA system. The peak height for 6.0 μM PHMB with the SPE columns was compared with that without those columns, and the concentration of adsorbed PHMB in the columns was measured from the calibration curve for PHMB without the columns.

#### **4. Conclusions**

A fluorescence optode based on an L-RB film for the determination of PHMB was fabricated. The response of the optode shows a good linear response between fluorescence intensity and the concentration of PHMB with the concentration range of 0 to 8.0 μM. The application of the L-RB film optode was made for the determination of PHMB in the CLDs. The procedures of the proposed method based on the L-RB film optode is much simpler than those of the conventional determination methods without using very toxic reagents. The L-RB film is easily fabricated with relatively low cost. The present method will be very promising for the quality control of CLDs. The L-RB film was used as a reversible film for the FIA of AS [16]. Therefore, the L-RB film will be used as a reversible film for the determination of PHMB. We continue the research regarding the FIA of PHMB using the optode detector based on L-RB film as a reversible film.

**Author Contributions:** Data curation, A.F., Y.H., and T.K.; Supervision, T.M.; Writing—original draft, T.M.; Writing—review and editing, T.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** We would like to appreciate Toshiaki Hattori (Toyohashi University of Technology) for the donation of PHMB very much.

**Conflicts of Interest:** The authors declare no conflict of interest.
