Micro-Capillary Coatings Based on Spiropyran Polymeric Brushes for Metal Ion Binding, Detection, and Release in Continuous Flow

Micro-capillaries, capable of light-regulated binding and qualitative detection of divalent metal ions in continuous flow, have been realised through functionalisation with spiropyran photochromic brush-type coatings. Upon irradiation with UV light, the coating switches from the passive non-binding spiropyran form to the active merocyanine form, which binds different divalent metal ions (Zn2+, Co2+, Cu2+, Ni2+, Cd2+), as they pass through the micro-capillary. Furthermore, the merocyanine visible absorbance spectrum changes upon metal ion binding, enabling the ion uptake to be detected optically. Irradiation with white light causes reversion of the merocyanine to the passive spiropyran form, with simultaneous release of the bound metal ion from the micro-capillary coating.


Table of Contents:
S1: polySP polymeric brushes functionalised micro-capillary S2: Set-up for absorbance measurements of micro-capillaries S3: Photo-induced binding and releasing of metal ions S4: Videos S5: References S1. polySP polymeric brushes functionalised micro-capillary Figure S1. Schematic representation of the polySP polymeric brush structure and functionalised micro-capillary.  Figure S3. Set-up used to study the absorbance spectra of the micro-capillary when M 2+ solutions (in ACN) are passed through the micro-capillary in continuous flow. The set-up is composed of a two fiber-optic light guides connected to a light source and a Miniature Fiber Optic Spectrometer (USB4000, Ocean Optics) and aligned using a cross-shaped cell. The M 2+ solution (in ACN) is passed through the microcapillary using a syringe pump. Figure S4. Microscopy photos of a section of a micro-capillary modified with spiropyran polymer brushes (polySP) before (left) and after irradiation for 20 s with UV light (middle) followed by the addition of Co 2+ (right). The micro-capillary returns to colourless (due to the conversion of the polyMC to polySP) after irradiation with white light for 1 min, resulting in the release of Co 2+ ions.

S3. Photo-induced binding and releasing of metal ions
In order to prove the release of the bound metal ion from the SP-polymer brushes coated micro-capillary through irradiation with white light, the release of metal ion was demonstrated in the case of Co 2+ through detection post modified micro-capillary using a chelating reagent, 4-(2-pyridylazo)resorcinol (PAR). PAR can coordinate to metal ions through a heterocyclic nitrogen group, azo group, and o-hydroxyl group, as shown in Figure   Firstly, the absorbance spectra of the chelating reagent (PAR) and its Co 2+ complex were recorded ( Figure S6) by passing a solution of PAR (1 mM) and PAR-Co 2+ (PAR: Co 2+ 1:1) through an unmodified glass micro-capillary at 2 µL min -1 . The spectra ( Figure S6) show the typical absorbance bands corresponding to PAR (black) and PAR-Co 2+ (red). The absorbance maximum for PAR-Co 2+ was recorded at ~ 510 nm. Figure S6. Absorbance spectra of the chelating reagent (PAR) and its Co 2+ complex. For the detection of the photo-released Co 2+ , the previous set-up ( Figure S3) was modified ( Figure S7) to include the injection of Co 2+ solution in ACN (1 mM), and the following steps were undertaken: 1. The pump (left) was turned on (flow rate = 20 µL min -1 ; mobile phase = ACN).
3. The polySP modified micro-capillary was irradiated with UV light for 20 s.
4. Co 2+ solution (1 mM) from the injection loop was injected in the system at a flow rate of 20 µL min -1 for approximately 5 min.
5. When all the expected Co 2+ solution left the detection area, both pumps (ACN and PAR) were turned OFF and the while light was turned ON.
6. After about 5 min, both pumps (ACN and PAR) were turned back ON.
7. The absorbance at λ max specific for PAR-Co 2+ (510 nm) was recorded during the whole experiment (steps 1-6) and plotted in Figure S8.

Micro-syringe pump
It is expected that, after the irradiation of the micro-capillary with white light (step 5), the Co 2+ ions will be released and then, with both pumps turned ON, the two confluent flows will react and PAR-Co 2+ will be formed. When reaching the detection area, PAR-Co 2+ will generate a change in the absorption spectra, generating a new absorbance band at 510 nm. This absorbance band ( Figure S8) was recorded during the experiment (steps 1 to 6) and shows an increase in the absorbance band at 510 nm when both the PAR flow (step 2) and Co 2+ flow (step 4) are turned ON. When the Co 2+ flow is turned OFF (step 5), a decrease in the band at 510 nm is observed until this reaches an absorbance of ~0 a.u. indicating that all Co 2+ has exited the detection area. Following this, the PAR flow is also switched OFF and the SP-M polymeric brushes functionalised micro-capillary is irradiated with white light for 5 minutes. Finally, the ACN and PAR flows are switched ON. This causes an increase in the band at 510 nm ( Figure S8, step 6) indicating that indeed Co 2+ was released upon white light irradiation from the modified micro-capillary. Figure S8. Absorbance at 510nm recorded on a USB400 spectrometer using the setup depicted in Figure S7 during experimental steps 1-6. The increase of the absorbance band centred at 510nm indicates the presence of PAR-Co 2+ complex.

S4. Videos
Video S1 shows in real time the colour change of the spiropyran norbornene monomer crystals under different illumination conditions. In the video, the UV light was turned ON at 0:45 and switched OFF after ~ 2 min (time 2:49), followed by ~ 3 min of white light irradiation (white light ON at 5:13 and switched OFF at 8:21). The video was recorded on a benchtop Aigo digital Microscope GE5, at a magnification of 180x.