one. Article overview
From medicine to environmental science, gas sensors are the core of various fields, and the demand for gas sensors is rapidly expanding. However, in the face of complex gas samples, how to maintain high sensitivity while performing multiple detections is still a problem that plagues researchers. By introducing Ti3C2Tx MXene into a three-dimensional (3D) microfluidic gas sensor with a transferable SERS substrate, the article demonstrates a powerful gas sensor with both multi-channel detection capability and high sensitivity. The use of MXene enables the sensor to have a generally high adsorption efficiency for various gases, and the generation of in-situ gas vortices in the complex nanostructure prolongs the residence time of the molecules in the sers active zone, which leads to an increase in sensitivity. In the proof-of-concept experiment, based on the inherent SERS signal of gas molecules, the detection limit (LOD) of three typical volatile organic compounds (VOCs) reached 10-50 ppb. In addition, a well-designed periodic three-dimensional structure solves the repeatability problem of general SERS substrates. In addition, the use of classical least square analysis (CLS) reveals the detailed composition of the gas mixture, with an average accuracy of 90.6%. In addition, a color bar code based on CLS results was developed to visually read the complex composition of the sample.

Two, graphic guide

Figure 1. Preparation and characterization of a three-dimensional SERS substrate based on colloidal assembly. (a-c) PS substrate preparation schematic diagram and corresponding SEM image. The scale bars of the inserted SEM images are all 10 μm. (d-e) 3D and 2D AFM images of PS substrate. The height color bar of the two-dimensional image is 0 ~ 2500nm. (f) The cross-sectional height diagram of (e) is marked with a solid white line. (g i) Schematic diagram and SEM image of the preparation process of bimetallic nanocubes. The scale bar of SEM images is 100 nm. (j) A schematic diagram of bimetallic nanocubes loaded on a PS substrate, and (k) the corresponding SEM image. The scale bar is 10 μm. The scale bar of the inserted image is 1 μm.

Figure 2.3D SERS substrate transfer. (a c) Schematic diagram of the 3D SERS substrate to be transferred. (d) SEM image of the transfer substrate. The scale bar is 10 μm, and the scale bar of the inserted image is 1 μm. (e) Transfer the patterned substrate of SEU with laser cutting tape. The scale bar in the digital photo is 2 mm. (f) Photo of herringbone chip with transfer substrate. The inserted image is an optical micrograph of the substrate. The scale bar on the left is 2mm, and the scale on the right is 5 μm. (g) SERS mapping result of R6G. The color bar ranges from 0 to 550, according to the peak intensity at 1360 cm 1. (h) R6G spectra obtained on different chips. (i) Numerical simulation of electromagnetic field in a single pit with bimetallic nanowires. The excitation wavelength is 632.8 nm.

Figure 3. Synthesis and characterization of monolayer Ti3C2Tx MXene. (a) Schematic diagram and image of single-layer Ti3C2Tx MXene preparation. The scale bars inserted into the SEM image are 1 mm, 5 μm, and 1 μm, respectively. (d) The 2D AFM image of a single layer of Ti3C2Tx MXene and (e) the height change of the MXene film along the solid white line. The height color bar is 1 ~ 3nm. (f) Elemental mapping result of Ti3C2Tx MXene measured by HRTEM and (g) EDX spectrum. The carbon signal comes from Ti3C2Tx MXene and the carbon film on the copper mesh.

Figure 4. Detection of typical volatile organic compounds. (a) Benzaldehyde (e) DNT and (i) indole are detected by SERS-Vortexene chip under the condition of 100 ppm. The SERS intensities of the gases (b) benzaldehyde, (f) DNT and (j) indole at different concentrations were determined based on their peaks at 1004, 1340 and 756 cm 1 respectively. The experimental data are all fitted with s-type functions. (c) Full spectrum two-dimensional superimposed images of benzaldehyde, (g) DNT and (k) indole at different concentrations. (d) Color bar code of benzaldehyde, (h) DNT and (l) indole at a concentration of 100 ppm. The color bar ranges from 0 to 100 ppm. The line width is proportional to the relative peak intensity.

Figure 5. SERS barcode of VOCs mixture extracted by CLS method in multiplex detection. (a) Schematic diagram of multiple VOCs gas detection. (b) The right y-axis of the histogram is the relative error between the intensity calculated by CLS and the intensity calculated by the standard curve according to the preset concentration. On the left y-axis, the covariances of the 14 groups of CLS fitting are represented by scatter points. (c) SERS spectrum of sample ternary gas mixture. (d) The spectrum of each evaporated compound calculated from the CLS results. (e) The color bar code of each work. According to the standard detection curve, the calculated concentration is converted into concentration. The barcode of the mixture is the result of the superposition of three independent barcodes. The color bar ranges from 0 to 100 ppm.
3. Full text summary

In summary, by combining the transferred three-dimensional SERS substrate with the ultra-flexible Ti3C2Tx MXene, the simultaneous and direct multi-channel gas sensing of the SERS-vortexene chip has been realized, showing high sensitivity, multi-channel detection ability and good performance. Excellent sensing performance such as reproducibility. Control experiments and theoretical simulations prove that the gas vortex generated by the three-dimensional structure in the microfluidic channel prolongs the remaining time of gas molecules and significantly improves the sensitivity. In addition, the excellent adsorption capacity of MXene materials for VOCs also plays a key role in improving detection sensitivity. On the other hand, in order to achieve multiple detection capabilities, a visual SERS barcode was developed in conjunction with the CLS method, which helps to make full use of the multiple detection capabilities of SERS technology. Combining these methods, the chip recognizes benzaldehyde, DNT and indole, with multiple detection LOD as low as 10 ppb and error as small as 9%, which solves the long-term trade-off between multiple detection capabilities and sensitivity. It is worth mentioning that these methods can also be used alone to improve detection performance. In addition, based on the SERS fingerprint spectrum and the universal adsorption capacity of MXene, the SERS-vortexene chip can fully support the identification of a variety of VOCs and their mixtures through the establishment of a spectral library. The author expects that the proposed SERS-Vortexene chip will help further research on ultra-flexible MXene and sers-microfluidic technologies, and will help develop advanced gas sensors with balanced versatility and sensitivity.
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