A recyclable polyoxometalate-based supramolecular chemosensor for efficient detection of carbon dioxide

A new type of supramolecular chemosensor based on a polyoxometalate and a block copolymer was constructed for the qualitative and quantitative detection of CO2.


Experimental Section
Materials and Instruments. Na 9 DyW 10 O 36 was prepared as described by Caronado. [S1] Block copolymer PEO 114 -b-PDMAEMA 16 (PDI=1.2) was purchased from Polymer Source, Canada. The deprotonation of PEO 114 -b-PDMAEMA 16 was carried out by adjusting its aqueous solution with NaOH to pH = 12, and then extracting with CH 2 Cl 2 , finally precipitated in n-hexane/ethyl ether (v/v, 7/3). All aqueous solutions of polyoxometalate and polymers were prepared using ultrapure water from a Millipore Milli-Q system. CO 2 (99.99%) and argon (99.999%) were purchased from Beijing Haike Yuanchang general aeriform Co. Ltd and used as received.
1 H NMR spectra were recorded on a Bruker 400 AVANCE Ⅲ spectrometer operating at 400.23 MHz, using D 2 O as solvent. TEM images were obtained on a JEM 2100 instrument operating at an acceleration voltage of 200 kV. The specimen was prepared by drop-casting the sample solution onto a carbon-coated copper grid for a few minutes. Excess solution was blotted away with a strip of filter paper, and the sample grid was dried in air. Photoluminescence measurements were performed on an FLS 920 Steady State & Time-resolved Fluorescence Spectrometer (Edinburgh Instruments Ltd.). Small-angle X-ray scattering (SAXS) measurements were performed using a SAXSess (Anton Paar) equipped with Kratky block-collimation system. The scattering pattern was recorded on an imaging plate (IP) with a pixel size of 42.3×42.3 μm 2 which extended to the high-angle range (the q range covered by the IP was from 0.06 to 29 nm -1 ). We carefully loaded the DyW 10 /PEO-b-PDMAEMA solution samples before and after exposure to CO 2 and upon Ar bubbling that removes CO 2 for SAXS measurement into a quartz capillary with a diameter of 1 mm at ambient temperature. The scattering curve of pure water filled in the same capillary was measured as the background. Background subtraction and desmearing were conducted by using SAXSquant 3.6 software. The pairdistance distribution function (PDDF) of scattering curves was calculated using the generalized indirect Fourier transform (GIFT) [S2] program included in the SAXSess software package. The quantum yields of the complex solution were determined by the relative method with tryptophan as standard, excited at 280 nm.

PDMAEMA 16
The pH change was measured using a pH-meter (Mettler). The protonation equation of DMAEMA can be expressed as follows: The degree of protonation (δ) of DMAEMA in diblock copolymer PEO-b-PDMAEMA mainly depends on its pK a . As many reports mentioned, the pK a of PDMAEMA was approximately 7.4. [S3] Therefore, the quantity of the protonated DMAEMA in every PEO-b-PDMAEMA chain at different pHs can be calculated according to eqs. (1), (2), and (3): (1) complex solution was about 7.2, and the δ can be calculated to be 61%. Upon alternative treatment with CO 2 and Ar, the pH of DyW 10 /PEO-b-PDMAEMA complex solution can be switched between 4.8 and 6.9. Thus, δ can be calculated to be 99.8% at pH 4.80 and 75.1% at pH 6.92.

Fluorescence lifetime analysis
The fluorescence decay measurements were performed using a time-correlated single-photon counting technique, in which the fluorescence decay was monitored at 476 nm.
The fluorescence decays were fitted by the following equation: where f i represents fractional contribution to the total fluorescence decay. [S4] In DyW 10 aqueous solution (0.2 mg mL -1 ), the DyW 10 luminescence showed first-order decay with a lifetime 5.2 μs. With the addition of PEO-b-PDMAEMA, the decay curves cannot be fitted by a single exponential but at least three lifetimes, where the lifetime was much longer. In addition, excellent first-order decay was turned up as CO 2 gas bubbled into the DyW 10 aqueous solution in the presence of PEO-b-PDMAEMA, where the lifetime is much longer, as to 57.5 μs. Interestingly, after degassing CO 2 dissolved in the solution by purging Ar, the decay lifetimes was restored to three lifetimes, that is, τ 1 ≈ 4.5 μs, τ 2 ≈ 20 μs, and τ 3 ≈ 60 μs, which is similar to the initial state of DyW 10 /PEO-b-PDMAEMA complex. The fitting parameters of each state are summarized in Table S1.

Calculation the number of water molecules (q) coordinated to Dy 3+ ion
The values of decay lifetime of DyW 10 /PEO-b-PDMAEMA complex in H 2 O solution and its corresponding D 2 O solution were recorded. According to the literature, [S5] the number of water molecules q coordinated at the dysprosium center can be calculated by using the following equation:  