Extrusion Printing of Surface‐Functionalized Metal‐Organic Framework Inks for a High‐Performance Wearable Volatile Organic Compound Sensor

Abstract Wearable sensors hold immense potential for real‐time and non‐destructive sensing of volatile organic compounds (VOCs), requiring both efficient sensing performance and robust mechanical properties. However, conventional colorimetric sensor arrays, acting as artificial olfactory systems for highly selective VOC profiling, often fail to meet these requirements simultaneously. Here, a high‐performance wearable sensor array for VOC visual detection is proposed by extrusion printing of hybrid inks containing surface‐functionalized sensing materials. Surface‐modified hydrophobic polydimethylsiloxane (PDMS) improves the humidity resistance and VOC sensitivity of PDMS‐coated dye/metal‐organic frameworks (MOFs) composites. It also enhances their dispersion within liquid PDMS matrix, thereby promoting the hybrid liquid as high‐quality extrusion‐printing inks. The inks enable direct and precise printing on diverse substrates, forming a uniform and high particle‐loading (70 wt%) film. The printed film on a flexible PDMS substrate demonstrates satisfactory flexibility and stretchability while retaining excellent sensing performance from dye/MOFs@PDMS particles. Further, the printed sensor array exhibits enhanced sensitivity to sub‐ppm VOC levels, remarkable resistance to high relative humidity (RH) of 90%, and the differentiation ability for eight distinct VOCs. Finally, the wearable sensor proves practical by in situ monitoring of wheat scab‐related VOC biomarkers. This study presents a versatile strategy for designing effective wearable gas sensors with widespread applications.


Synthesis of dye/UiO.
After dissolving 10 mg of the dye in 50 mL of ethanol, 50 mg of UiO-66(Zr) was introduced into the solution.The mixture was stirred at room temperature for 12 h.The resulting sample was obtained by centrifuging at 9,000 r.p.m for 15 min, followed by three ethanol washes, and finally dried under vacuum at room temperature for 12 h.

Characterization.
Scanning electron microscopy (SEM) images were carried out on a Zeiss G300 scanning electron microscope (Zeiss, Germany).High-resolution transmission electron microscopy (HRTEM) images were observed using a JEOL JEM-2100Plus transmission electron microscope (JEOL, Japan) at an accelerating voltage of 200 kV.Energy-dispersive X-ray spectroscopy (EDS) element mapping images were conducted on an FEI Tecnai G2 F20 S-TWIN transmission electron microscope (FEI, USA) equipped with Oxford X-MAX 80T EDS detector (Oxford Instruments, UK).Powder X-ray diffractometer (PXRD) patterns were recorded on a Bruker D8 Advance diffractometer (Bruker, Germany) (Cu Kα X-ray radiation, λ = 1.54 Å).Each PXRD pattern was acquired at a voltage of 40 kV and a current of 40 mA.
Fourier transform infrared spectroscopy (FT-IR) spectra were measured using a Thermo Scientific NICOLET iS50FT-IR spectrometer (Thermo Fisher Scientific, USA) with a scanning range of 500-3500 cm −1 and resolution of 4 cm −1 .The static water contact angles were measured using a Biolin Scientific Theta Lite instrument (Biolin Scientific, Sweden).The tensile testing of the printed sensor was performed on a universal testing machine (UTM2102, SUNS, Shenzhen, China).

Testing of the humidity-resistance capacity of the sensor arrays.
The colorimetric sensors were placed in the chamber with different RH for 30 min.The RH of the gas chamber is adjusted by mixing the proper proportion of dry and wet N2 using digital mass-flow controllers (CSC200-C, Sevenstar, Beijing Sevenstar Electronics Co., Ltd., China) and measured by a digital humidity sensor (Jianda Renke, Shandong Renke Control Technology Co., Ltd., China).The images of the sensor array before and after exposure were recorded using a scanner (V600, EPSON, Japan).

Inoculation of wheat leaves.
Wheat seedlings (Jimai 22) were cultivated in a greenhouse at 25 °C under 12 h of light per day.
A typical Fusarium graminearum (F.graminearum) wild-type strain PH-1 (NRRL 31084) was cultured on potato-dextrose agar (PDA) plate culture media at 25 °C for 4 d.The wheat plant at the stage of 10 d after sowing was inoculated by spraying F. graminearum sporangia solution (~50,000 sporangia ml −1 ) on the whole plant.The inoculated plants were put in an incubator (25 °C, 80 % RH) under 12 h of light per day.The healthy wheat plants were treated as the same operation, used as a control.Euclidean Distance (a.u.) Relative Humidity (%)  The dye uptake was calculated from the following equation: (C0: initial concentration of dye in ethanol solution (mg/L); Cad: the concentration after adsorption (mg/L); m: the mass of adsorbent MOF (mg); V is the volume of the solution (L)).As the nozzle size and N2 pressure increase, the volume of ink ejected rises, resulting in progressively thicker films.Excessive ink ejection, however, can impede the uniform formation of the film.This phenomenon arises from the higher surface tension of larger ink droplets, which slows down their diffusion rate on the substrate.Consequently, the prolonged evaporation and solidification process complicates the formation of a uniform and stable thin film.To achieve a uniform film, the optimized printing parameters are the nozzle size of 32 G and a pressure of 5 Kpa.

Figure S3 .
Figure S3.(a) UV-vis calibration curve of bromophenol blue.Inset: structure and molecular size of bromophenol blue.(b) UV-vis spectra of the pristine dye solution and the supernatant after UiO adsorption under different adsorption time.To ensure that the measured values do not exceed the measurement range, all test solutions were diluted to 1/4 of their original concentration.(c) Dye-loading amount after different adsorption time.(d) The ED values of corresponding dye/UiO@PDMS-150 to 10 ppm of 1-octene-3-ol.

Figure S4 .
Figure S4.Optical images of (a) PDMS/DUP (70% wt) ink and (b) squeezing ink through a syringe, showing appropriate viscosity and satisfied rheological properties.

Figure S5 .
Figure S5.Path of the printing nozzle for preparing the hybrid film (a square with a side length of 3 mm) designed using MuCAD software.

Figure S6 .
Figure S6.Optical images of PDMS/DUP (70% wt) films (a square with a side length of 3 mm) printed using extrusion nozzles of different sizes and under various N2 pressures.The corresponding inner diameter for standard sizes of 32 G, 30 G, and 28 G are 100 μm, 150 μm, and 170 μm.Under varying nozzle sizes and extrusion N2 pressures, the prepared films exhibit different levels of quality.As the nozzle size and N2 pressure increase, the volume of ink ejected rises, resulting in progressively thicker films.Excessive ink ejection, however, can impede the uniform formation of the film.This phenomenon arises from the higher surface tension of larger ink droplets, which slows down their diffusion rate on the substrate.Consequently, the prolonged evaporation and solidification process complicates the formation of a uniform and stable thin film.To achieve a uniform film, the optimized printing parameters are the nozzle size of 32 G and a pressure of 5 Kpa.

Figure S10 .
Figure S10.Water contact angle images of PDMS substrate and printed film with different dye/UiO@PDMS particle loading.

Figure S12 .
Figure S12.(a) cross-section SEM image of PDMS/DUP (70%wt) film by spin coating.(b) optical image showing the tensile testing.The test sample is prepared by spin-coating of PDMS/DUP (70%wt) ink on a PDMS substrate (the thickness of substrate: 50 μm).(c) Amplifying photographs of the samples in (b) under various strains.The crack appeared when the strain was 10%.

Figure S13 .
Figure S13.UV-vis calibration curve of dye solution, and UV-vis spectra of dye solutions before (red line) and after (grey line) 12-h exposure to UiO-66 (Zr) at room temperature.The structure and molecular size of corresponding dyes were inserted.

Figure S14 .
Figure S14.Optical images of the printed sensor under various strains from 0% to 50%.

Figure S16 .
Figure S16.PCA plot with the first three principal componets of 8 plant VOCs and control at (a) 1 and (b) 0.5 ppm, based on three independent experiments of 20-min response to VOCs.

Figure 17 .
Figure 17.Color differential patterns of the sensor array in response to various generic VOC vapors, including acetone, ethanol, methanol, chloroform, and toluene.The RGB color range of 3-10 was expanded to 0-255 for display purposes.

Figure S18 .
Figure S18.Color differential patterns of the sensor array to VOC mixture and the corresponding single VOC.The RGB color range of 3-10 was expanded to 0-255 for display purposes.

Figure S19 .
Figure S19.Response of the printed sensor to 30-min water vapor exposure under different RH.

Figure S20 .
Figure S20.Optical images of the wheat leaf surface before and after applying the sensor.(a) Photo of the wheat leaf before measurement, (b) attached with the sensor, and (c) after measurement.There is no observed physical damage on the wheat leaf after a 30-minute measurement.

Table S1 .
The corresponding dye molecules in each spot of sensor arrays.

Table S2 .
Gas sensors utilizing MOF-based thin film.