Materials
Bovine serum albumin (BSA), 2-methylimidazole, 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), dimethyl sulphoxide (DMSO), ethanol, ethylene glycol, tertiary butanol, Triton X-100, fluorescein isothiocyanate (FITC), and rhodamine B were purchased from Sigma-Aldrich. Zinc nitrate hexahydrate, zinc acetate, hydrogen peroxide (H2O2) were purchased from Alfa Aesar. Cytochrome c (Cyt c) from equine heart and horseradish peroxidase were purchased from Hoffmann-La Roche. Other chemicals are of analytical grade.
Printing procedure was carried out on a piezoelectric ink-jet printer Epson ME-10. The four ink cartridges were filled with self-formulated enzyme or precursors solutions; the graphics files to be printed were prepared by Adobes Photoshops CC software in CMYK color space. Whatman No. 1 filter paper was purchased from Sigma-Aldrich. Polyvinylchloride (PVC) film and poly(ethylene terephthalate) (PET) film were produced by Empero Co., LTD. and hydrophilic printing film was produced by FULEJET Co., LTD.
Preparation of ink solutions
Four different formulations of inks (Additional file 1: Table S1) were prepared to obtain an appropriate surface tension and viscosity in the range of 30–50 mN/m and 2–5 mPa s, which is required for a smooth and continuous ink-jet printing. The viscosity measurement was carried out on a Physica MCR 301 rheometer under a shearing rate at 100/s at 25 °C. Inks 1–3 were used to dissolve and print precursors of ZIF-8 at required concentrations, and Ink 4 was used to dissolve and print protein and other molecules at required concentrations.
The inks were prepared before printing and filtered through a 0.45-μm membrane prior to filling in the color cartridges. After each printing experiment, an automatic washing produce of the printer was conducted to avoid the block of nozzle by aggregates of metal ions, protein molecules, or other molecules.
For the investigation of formulation of inks, Inks 1–3 were used to dissolve zinc nitrate hexahydrate (Zn2+) and 2-methylimidazole (2-MeIM) at a concentration of 125 mmol/L and 1 mol/L, respectively. The ink solution containing Zn2+ was loaded in the C cartridge and the ink solution containing 2-MeIM was loaded in the M cartridge. The graphics files to be printed were prepared by Adobes Photoshops CC software in CMYK color space by fixing C = 100% and M = 100%. The patterns to be printed were 3 cm × 3 cm squares. The printing substrate was filter paper.
Investigation of different printing substrates and printing times
For the investigation of printing of pure ZIF-8 on different substrates including filter paper, PVC film, PET film, and hydrophilic film, Ink 1 was used to dissolve zinc nitrate hexahydrate (Zn2+) and 2-methylimidazole (2-MeIM) at a concentration of 125 mmol/L and 1 mol/L, respectively. The ink solution containing Zn2+ was loaded in the C cartridge and the ink solution containing 2-MeIM was loaded in the M cartridge. The graphics files to be printed were prepared by Adobes Photoshops CC software in CMYK color space by fixing C = 100% and M = 100%. The patterns to be printed were 3 cm × 3 cm2. Various times of printing (interval time between each printing was 5 min) were achieved by repeating the printing on the same pattern.
Investigation of precursors concentrations and printing ratios
For the investigation of concentrations of Zn2+ and 2-MeIM, Ink 1 was used to dissolve zinc nitrate hexahydrate (Zn2+) and 2-methylimidazole (2-MeIM) at a concentration of 125 mmol/L (or 62.5 mmol/L) and 1 mol/L (or 0.5 mol/L), respectively. The ink solution containing Zn2+ was loaded in the C cartridge and the ink solution containing 2-MeIM was loaded in the M cartridge. The graphics files to be printed were prepared by Adobes Photoshops CC software in CMYK color space by fixing C = 100% and M = 100% (Zn2+/2-MeIM molar ratio 1:8), or fixing C = 100% and M = 50% (Zn2+/2-MeIM molar ratio 1:4). The patterns to be printed were 3 cm × 3 cm squares. The printing substrate was filter paper.
Printed protein-induced formation of ZIF-8
Ink 4 was used to dissolve BSA at a concentration of 50 mg/mL. Then the protein ink was loaded in Y cartridge and printed on PET film with a predesigned pattern. After drying the film with printed protein pattern at room temperature for 4 h, the film was immersed in a solution containing 20 mmol/L of zinc acetate and 80 mmol/L 2-methylimidazole solution allowing reaction for 8 h. After washing with DI water to remove unreacted metal ions and organic ligands, the film was dried at room temperature for 2 h. The graphics files to be printed were prepared by Adobes Photoshops CC software in CMYK color space by fixing Y = 100%.
Printing Cyt c–ZIF-8 composites
Ink 1 was used to dissolve zinc nitrate hexahydrate (Zn2+) and 2-methylimidazole (2-MeIM) at a concentration of 125 mmol/L and 1 mol/L, respectively. Ink 4 was used to dissolve Cyt c (or Cyt c-FITC) at a concentration of 4 mg/mL. Then inks containing Zn2+, 2-MeIM, and protein were loaded in C, M, Y cartridges and printed on hydrophilic film with a predesigned pattern. With this order of cartridges, protein molecules can induce the formation of ZIF-8. The graphics files to be printed were prepared by Adobes Photoshops CC software in CMYK color space by fixing C = 100%, M = 100%, and Y = 100%.
Ink 4 was used to dissolve rhodamine B at a concentration of 4 mg/mL and was loaded in K cartridge. The graphics files to be printed were prepared by Adobes Photoshops CC software in CMYK color space by fixing C = 100%, M = 100%, and Y = 100% and adjusting the percentage of K from 0 to 100% (0, 20, 40, 60, 80, 100%).
Printing Cyt c–ZIF-8 composites on filter papers to construct testing strips
Ink 1 was used to dissolve zinc nitrate hexahydrate (Zn2+) and 2-methylimidazole (2-MeIM) at a concentration of 125 mmol/L and 1 mol/L, respectively. Ink 4 was used to dissolve Cyt c at a concentration of 4 mg/mL. Then inks containing Zn2+, 2-MeIM, and protein were loaded in C, M, Y cartridges and printed on filter papers as round patterns with diameters of 6 mm. The graphics files to be printed were prepared by Adobes Photoshops CC software in CMYK color space by fixing C = 100%, M = 100%, and Y = 100%.
Each round piece was utilized as a testing strip. 10 μL of ABTS (2.8 mg/mL in water) and 10 μL of H2O2 solution with different concentrations were dropped on the round pieces. Once the round strips turned green after 2 min, the color intensity (gray value) was analyzed using the Image J2x software.