Electronic Supplementary Information

A colloidal water-stable MOF as a broad-range fluorescent pH sensor via post-synthetic modification

J. Aguilera-Sigalat*a and D. Bradshaw*a

a School of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, UK. E-mail: D.Bradshaw@soton.ac.uk Tel: +44(0)23 8059 9076 J.Aguilera-Sigalat@soton.ac.uk

1. Materials

Zirconium chloride (ZrCl4), 2-aminoterephthalic acid (bdc-NH2), sodium nitrite (NaNO2) and 1-methylindole were purchased from Sigma-Aldrich. DMF, HCl, EtOH, acetic acid and acetone were purchased from Fisher. All chemical were used without further purification.

2. Characterization

X-Ray diffraction data were collected on a Bruker D2 phaser in the angular range 2θ = 5-40o employing a Ni Kβ filter (detector side) producing Cu (Kα1/Kα2) radiation.

Photoluminescence: Steady-state emission spectra were acquired using an Agilent Eclipse Fluorescence Spectrophotometer with a Xenon flash lamp.1H-NMR spectra were acquired using a Bruker DPX400 FT-NMR spectrometer. 5 mg of dry samples were digested in 700 µL of DMSO and 5 µL of HF (48 % water solution) with sonication

Scanning Electron Microscopy measurements were made on a JEOL JSM 6500F field-emission scanning electron microscope at an accelerating voltage of 10 kV. The sample for SEM measurements was prepared by firstly placing a drop of microsphere suspension in absolute ethanol on a carbon paste attached on an aluminium substrate, being dried under vacuum overnight, then sputter-coated with a thin conductive gold layer.

BET: N2 adsorption/desorption isotherms were measured at 77 K using a Micromeritics 3-Flex Surface Characterization Analyzer after the sample was first degassed under vacuum at 170 °C overnight. Surface areas were determined by the BET method in an appropriate pressure range.

Electronic Supplementary Material (ESI) for ChemComm.This journal is © The Royal Society of Chemistry 2014

UV-visible spectra: Samples were dispersed in EtOH and digested with HF, and the spectra were carried out on a Shimadzu 2700.

TGA: Thermogravimetric analysis (TGA) was performed using a TG 209 F1 Libra (Netzsch) and the sample was heated from room temperature to 900 oC at a rate of 10 oC min-1 under an air atmosphere.

3. Synthesis details

Synthesis of UiO-66-NH2 1

740 mg of ZrCl4 (3.17 mmol) were dissolved in 183 mL of DMF (2377.5 mmol). Then, 574 mg of 2-aminoterephthalic acid (3.17 mmol) were added and the mixture was sonicated for 10 min before heating in oven at 120 oC for 24 h. After cooling to room temperature, the obtained precipitate was centrifuged (10000 rpm, 12 min) and washed three times with DMF and three times with EtOH. Finally, the yellow powder was dried at 100 oC for 24 before further characterization.

Synthesis of UiO-66-NH2 with acetic acid as modulator 2

740 mg of ZrCl4 (3.17 mmol) were dissolved in a mixture of 183 mL of DMF (2377.5 mmol) and 5.46 mL of acetic acid (95.1 mmol). Then, 574 mg of 2-aminoterephthalic acid (3.17 mmol) were added and the mixture was sonicated for 10 min before heating in oven at 120 oC for 24 h. After cooling to room temperature, the obtained precipitate was centrifuged (10000 rpm, 12 min) and washed three times with DMF and three times with EtOH. Finally, the yellow powder was dried at 100 oC for 24 before further characterization.

Synthesis of UiO-66-NH2 with HCl 3

125 mg of ZrCl4 (3.17 mmol) were dissolved in a mixture of 10 mL of DMF and 1 mL of HCl, and this was sonicated for 20 min. Then, 134 mg of 2-aminoterephthalic acid (0.74 mmol) and 5 mL of DMF were added and the mixture was sonicated for another 20 min before heating in oven at 80 oC overnight. After cooling to room temperature, the obtained precipitate was centrifuged (10000 rpm, 12 min) and washed three times with DMF and three times with EtOH. Finally, the yellow powder was dried at 100 oC for 24 before further characterization.

Synthesis of UiO-66-N=N-ind 4

50 mg of UiO-66-NH2 (0.178 mmol, eq NH2) were dispersed in 20 mL of Milli-Q water and 13.5 mg of NaNO2 (0.196 mmol) was added. The mixture was cooled to 0 oC and, under vigorous stirring, 20 mL of HCl 0.03 M (0.6 mmol) was added dropwise. After two hours of reaction at 0 oC, 49 µL of 1-methylindole (0.392 mmol) was added. After a few minutes the reaction turned from yellow to orange. After allowing the mixture to warm to ambient, the mixture is stirred for 1 or 3 hours. The orange precipitate was centrifuged (10000 rpm, 12 min) and washed three times with H2O and three times with acetone. Finally, the powder was dried at 100 oC for 24 for further characterization.

Scheme 1. Reaction scheme for PSM of UiO-66-NH2 into UiO-66-N=N-ind

4. pH-dependence Fluorescence/UV studies

3 mg of UiO-66-NH2 or UiO-6-N=N-ind was dispersed in 6 mL of Milli-Q water by and sonicated for 10 min. 100 µL of this solution was added to the correspondent cuvettes containing 800 µL of Milli-Q water in the range of pH from 1 to 12 (adjusted using HCl or NaOH). The fluorescence spectra (λex = 300 nm) were recorded immediately.

5. Results and additional figures

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Figure S1. Nitrogen sorption isotherms (77 K) of as-synthesized UiO-66-NH2 (black), UiO-66-N=N-ind1h (red) and UiO-66-N=N-ind3h (blue).

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UiO-66-NH2 after soaking UiO-66-N=N-ind1h after soaking UiO-66-N=N-ind3h after soaking

Figure S2. Nitrogen sorption isotherms (77 K) after soaking for 2h of UiO-66-NH2 at pH=9 (black), UiO-66-N=N-ind1h at pH=11 (red) and UiO-66-N=N-ind3h at pH=12 (blue).

BET as-made BET after soaking % difUiO-66-NH2 1630 ± 10 1320 ± 10 - 19

UiO-66-N=N-ind1h 811 ± 4 840 ± 4 + 3.6UiO-66-N=N-ind3h 552 ± 4 537 ± 2 - 2.7

Table S1. Summary of BET (m2/g) data

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Figure S3. Solid state FTIR spectra of UiO-66-NH2 (black), UiO-66-N=N-ind1h (red), UiO-66-N=N-ind3h (blue) and 2-aminoterephthalic acid (pink). The amino-streching peaks in the 2-aminoterephthalic acid (3502 and 3386 cm-1) are clearly visible in the as-synthesized UiO-66-NH2 (3444 and 3332 cm-1), although slightly shifted. However, after modification, these peaks have all but vanished which strongly indicates successful PSM via the diazotisation reaction. Such disappearance of amino bands following PSM has also been observed in Chem Commun, 2013, 49, 10575

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Figure S4. Infrared spectra in solid state of UiO-66-NH2 (black), UiO-66-N=N-ind1h (red), UiO-66-N=N-ind3h (blue) in the finger print region.

Theoretical (%) Experimental (%)C 32.8 28.7H 2.0 3.3UiO-66-NH2

N 4.8 4.3C 45.6 50.2H 2.5 3.8UiO-66-N=N-ind3hN 8.5 7.4

Table S2. Theoretical (according to the fluorescent conversion, http://fluorine.ch.man.ac.uk/research/analyse2.php) vs. experimental elemental analysis of: UiO-66-NH2 and UiO-66-N=N-ind3h (~70 % conversion). Samples were prepared for analysis by solvent exchange (CHCl3) and heated to 60 °C in vacuo for several days. We and others have found it very difficult to remove all solvents from UiO-66 based frameworks to obtain wholly reliable analysis and we observe that even after heating UiO-66-NH2 at 190°C some DMF is still visible by 1H-NMR. By contrast, UiO-66-N=N-ind3h begins to decompose under these conditions as evidenced by a blackening of the solid, and consistent with the absence of a well-defined step in the TGA profile (figure S5 below). However, it is clear from the analysis that there is significantly more C and N following diazotisation.

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Figure S5. TGA data of the as-synthesized UiO-66-NH2, UiO-66-N=N-ind1h and UiO-66-N=N-ind3h. The shallow continually sloping TGA profiles of the indole-functionalised frameworks suggest these are not as thermally stable as the UiO-66-NH2 parent phase. However, it is clear that as degree of diazotisation increases the remaining ZrO2 residue decreases, consistent with a greater contribution of the organic linker to the composition.

Figure S6. ESI-MS of (top) UiO-66-NH2 digested in NaOH (positive ion): m/z 182.0158 (calc. 182.0448) and (bottom) UiO-66-N=N-ind digested in NaOH (positive ion): m/z 324.0924 (calc. 324.0979) corresponding to the indole functionalised amino-bdc.

MOF ppm of ZrUiO-66-NH2 10.96

UiO-66-N=N-ind3h 0.26

Table S3. ICP-OES analysis of UiO-66-NH2 and UiO-66-N=N-ind3h supernatants after soaking the frameworks at pH=12 for 2 hours. Conditions of the experiment: 20 mg of MOF in 20 mL of Milli-Q water adjusted to pH=12.

Figure S7. SEM images of UiO-66-NH2 (top) and UiO-66-N=N-ind1h (bottom).

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Figure S8. Exponential correlation between the fluorescence response of UiO-66-NH2 (λex = 350, λem = 428 nm) vs. pH.

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Figure S9. Normalized fluorescence emission of 2-aminoterephthalic acid vs. pH (λex = 300 nm).

Figure S10. 1H-NMR spectra of UiO-66-NH2 (black) and UiO-66-N=N-ind1h (red) upon framework digestion in DMSO-HF. The 1H-NMR of UiO-66-NH2 after digestion with HF shows the typical signal of the free ligand (bdc-NH2) with some impurities in the aromatic region. These impurity peaks have previously been reported by Cohen et al.,5 and seem to be associated only with the synthesis of UiO-66-NH2, since other MOFs in the family (UiO-66, UiO-66-Br, UiO-66-NO2) do not show these impurities. The assignments of the NMR peaks for UiO-66-N=N-ind1h were interpreted according to the presence of the new signals observed for the MOF-indole moieties. The peaks corresponding to amino-bdc are set to an integration of 1 (each corresponds to 1 proton) and all new signals were integrated individually: for UiO-66-N=N-ind1h these are all approx. 0.3 (see red spectrum above) with respect to amino-bdc, corresponding to a conversion of ~25%. This is in excellent agreement with the linear reduction in fluorescence with increasing functionalisation shown in figure 3 of the manuscript.

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Figure S11. Exponential correlation between the fluorescence response of UiO-66-N=N-ind1h (λex = 350, λem = 428 nm) vs. pH.

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Figure S12. PXRD pattern of UiO-66-N=N-ind1h after soaking for 2h at pH 11 (red) and the empty sample holder (black). The broad baseline in UiO-66-N=N-ind1h is due to the small sample holder used in the PXRD analysis of this sample.

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Figure S13. UV-visible spectra in ethanol of the bdc-N=N-ind ligand (black), UiO-66-NH2 digested in HF (blue) and UiO-66-N=N-ind3h digested in HF (red). After digesting UiO-66-NH2 in HF, 2-aminoterephthalic acid shows a band around 370 nm. A broad red shift band is observed in UiO-66-N=N-ind3h after digestion in HF from 375 to almost 450 nm, due to the azo-compound formation, associated with the higher level of the conjugated π system. (S. J. Garibay and S. M. Cohen, Chem. Commun., 2010, 46, 7700-7702) For comparison, the bdc-N=N-ind ligand is also shown.

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Figure S14. 1H NMR spectra of UiO-66-NH2 (red) and UiO-66-NH-COCH3 (black) upon framework digestion in DMSO-HF revealing 80% conversion of the amine groups to amides.

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Figure S15. PXRD pattern for UiO-66-NH-COCH3 after soaking at pH 12 for 2h.

Figure S16. 1H NMR spectra of UiO-66-NH2 (black) and UiO-66-N=N-ind3h (red) upon framework digestion in DMSO-HF. Assignments are treated as outlined for UiO-66-N=N-ind1h (figure S10) and you can clearly see these peaks are in an approx. 2.2:1.0 ratio with amino-bdc corresponding to a conversion of ~70%.

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Figure S17. Normalized fluorescence intensity at pH 10 of UiO-66-NH2, UiO-66-N=N-ind1h and UiO-66-N=N-ind3h of equimolar samples with respect to (unfunctionalised) 2-aminoterephthalic acid ligand (λex = 350 nm).

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Figure S18. Normalized fluorescence emission of UiO-66-N=N-ind3h vs. pH recorded in polystyrene cuvettes (λex = 300 nm). The clear emission band at 380 nm arises from the cuvette itself, and since this does not change with the nature of the solution can be used as a convenient system-based internal reference.

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Model ExponentialEquation y = y0 + A*exp(Reduced Chi-Sqr 0.01666Adj. R-Square 0.99603

Value Standard ErrorI (428) / I (380) y0 0.43404 0.05253I (428) / I (380) A 0.00163 5.87704E-4I (428) / I (380) R0 0.69441 0.03028

Figure S19. Normalized exponential correlation ratio between the fluorescence response of UiO-66-N=N-ind3h and pH by using the polystyrene cuvettes as internal system reference (λex = 300 nm).

𝐼428𝐼380

= (0.43 ± 0.05) + (0.0016 ± 0.0006) ∗ 𝑒(0.69 ± 0.03) ∗ 𝑝𝐻

Equation S1. Ratiometric response of the UiO-66-N=N-ind3h to the pH change obtained from figure S8.

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