**3. Conclusions**

We demonstrate the Ru(II) complex-modified POSS and its application for oxygen-resistant ECL system. By employing POSS, various beneficial effects were obtained. Electronic currents and ECL intensities were enhanced in the electro- and optical measurements, and *E*onset in CV was lowered compared to Ru(bpy)3<sup>2</sup>+. Moreover, ECL quenching by oxygen was suppressed in the Ru-POSS/TPrA system. From our mechanistic studies, adsorption of Ru-POSS to the ITO electrode, followed by efficient TPrA oxidation on the ITO electrode were suggested. Finally, oxygen quenching was compensated by generation of the TPrA radical. Our findings and materials could be a scaffold for developing future sensing technologies based on ECL, such as real-time monitoring for small molecular pollutants which it is difficult to detect with conventional environment sensors. Furthermore, compared to the conventional system, the oxygen-resistance can be improved by connecting to POSS at this stage. As mentioned in the introduction, POSS has significant properties for capturing hydrophobic molecules

under aerobic conditions. In combination with the unique capturing property of the POSS unit, improvement of sensitivity and selectivity for the target detection might be proposed.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4360/11/7/1170/s1, Figure S1: Structures of Ru(bpy)3 2+, Ru(bpy)2(dmbpy)<sup>2</sup>+ and TPrA, Figure S2: (a) UV−vis absorption and (b) PL spectra of 1.0 × 10−<sup>5</sup> M solutions containing Ru(bpy)3 2+, Ru-Model and Ru-POSS in H2O. The excitation light at λabs\_MLCT was used for PL measurements. Inset figure in (a) is the inset around the MLCT band. The concentration of Ru-POSS was based on the Ru(II) complex unit, Figure S3: Cyclic voltammograms of 0.10 mM Ru(bpy)3 2+ and/or 0.1 M TPrA in 0.20 M PBS bu ffer (pH 8.8) at ITO electrode at a scan rate of 100 mV/s (n = 3), Figure S4: Time courses of ECL in BE of 0.50 mM Ru(bpy)3 2+ and 100 mM TPrA in 0.20 M PBS bu ffer (pH 8.8) with deoxygenation (solid line) or aeration (dashed line) with an ITO electrode at 1.2 V vs. Ag/AgCl, Figure S5: The determination of onset potential (*E*onset). The onset potentials were determined from the intersection of the tangents between the baseline and the signal current, Figure S6: Cyclic voltammograms of 0.10 mM Ru(II)-containing materials in 0.20 M PBS bu ffer (pH 8.8) with an ITO electrode at a scan rate of 100 mV/s (n = 3). The concentration of Ru-POSS was based on the Ru(II) complex unit, Figure S7: Initial time courses of current in BE of 0.10 mM Ru(II) complexes and 100 mM TPrA in 0.20 M PBS bu ffer (pH 8.8) at ITO electrode at 1.2 V vs. Ag/AgCl (n = 3). The concentration of Ru-POSS was based on the Ru(II) complex unit, Figure S8: Time courses of ECL with 0.1 mM Ru(bpy)3 2+, 0.1 mM Amino-POSS and 100 mM TPrA in 0.20 M PBS bu ffer (pH 8.8) with Ar (solid line) and O2 bubbling (dashed line) with an ITO electrode at 1.2 V vs. Ag/AgCl, Figure S9: Time courses of ECL in BE of 100 mM TPrA in 0.20 M PBS bu ffer (pH 8.8) with the modified ITO electrodes at 1.2 V vs. Ag/AgCl, Figure S10: Cyclic voltammograms (solid lines) and corresponding ECL curves (dashed lines) at a scan rate of 100 mV/s with 0.1 mM Ru-POSS and 100 mM TPrA in 0.20 M PBS bu ffer (pH 8.8) with an ITO electrode (Red line) and 100 mM TPrA in 0.20 M PBS bu ffer (pH 8.8) with the modified ITO electrode (Blue line). The concentration of Ru-POSS was based on the Ru(II) complex unit, Figure S11: Time courses of currents with (a) Ru-POSS (b) Ru(bpy)3 2+ (c) Ru-Model in BE of 100 mM TPrA in 0.20 M PBS bu ffer (pH 8.8) with (solid line) or without 0.10 mM Ru(II) complexes (dashed line) at ITO electrode at 1.2 V vs. Ag/AgCl (n = 3). The concentration of Ru-POSS was based on the Ru(II) complex unit, Figure S12: Time courses of ECL in BE of 100 mM TPrA in 0.20 M PBS bu ffer (pH 8.8) at 1.2 V vs. Ag/AgCl with the modified ITO electrodes immersed into the solution containing 0.1 mM Ru-POSS (based on the Ru(II) complex unit) with 100 mM TPrA in 0.20 M PBS buffer with variable pH values (pH 5.2: orange line and pH 8.8: blue line), Figure S13: Time courses of electric currents with (a) Ru-POSS, (b) Ru(bpy)3 2+, (c) Ru-Model and (d) Ru(bpy)3 2+ + Amino-POSS in BE of 0.10 mM Ru(II) complexes and 100 mM TPrA in 0.20 M PBS bu ffer (pH 8.8) with Ar (solid line) and O2 bubbling (dashed line) with and ITO electrode at 1.2 V vs. Ag/AgCl. The concentration of Ru-POSS was based on a Ru(II) complex unit. The concentration of Amino-POSS in (d) was adjusted to the same as the POSS unit in (a), Figure S14: Residual rates of emission intensities of the photo-excited Ru(II) complexes in aerated solutions consist of 0.01 mM Ru(II) complexes and 0.1 M TPrA in 0.2 M PBS solution (pH 8.8). Excitation wavelengths were λabs\_MLCT Residual rates were calculated from the equation, *<sup>I</sup>*PL\_Air/*I*PL\_Ar, where *I*PL\_Air is the intensity at λemmax in aerated solutions and *I*PL\_Ar is in the hypoxic solutions, Figure S15: Cyclic voltammograms (solid lines) and corresponding ECL curves (dotted lines) of 0.1 mM Ru(II) complexes and 100 mM TPrA in 0.20 M PBS bu ffer (pH 5.2) with an ITO electrode at a scan rate of 100 mV/s (n = 3). The concentration of Ru-POSS was based on a Ru(II) complex unit, Table S1: Residual rates of maximum ECL intensities and onset increase rates in BE in the aerated solution compared to the hypoxic one, Table S2: *E*onset in various conditions. Scheme S1: The summary of the ECL mechanisms in the Ru(bpy)3 <sup>2</sup>+/TPrA system (TPrA-+ = Pr3N-+, TPrA- = Pr2NC-HCH2CH3, P1= Pr2N<sup>+</sup>CHCH2CH3).

**Author Contributions:** Conceptualization, K.T.; data curation, R.N., H.N. and M.G.; funding acquisition, K.T. and Y.C.; investigation, R.N. and H.N.; project administration, K.T.; supervision, K.T.; writing—original draft, R.N., M.G. and K.T.; writing—review and editing, Y.C.

**Funding:** This work was partially supported by the Kurita Water and Environment Foundation (for K.T.), a Grant-in-Aid for Scientific Research (B) (JP17H03067) and (A) (JP 17H01220) and for Scientific Research on Innovative Areas "New Polymeric Materials Based on Element-Blocks (No.2401)" (JP24102013).

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