CN111141718A - Application of two-dimensional titanium carbide nanosheet in Raman test, Raman test substrate and preparation method thereof - Google Patents

Abstract

The invention discloses an application of two-dimensional titanium carbide nanosheets in Raman testing, a Raman testing substrate and a preparation method thereof, wherein Ti is added₃AlC₂Adding the powder into an organic alkali aqueous solution, stirring and centrifuging to obtain Ti₃C₂Nanosheets; then adding Ti₃C₂Dispersing the nano sheets in water to form a colloidal solution; and then dripping the colloidal solution on a nonmetal carrier to obtain the Raman test substrate. By using in-situ modified titanium carbide nanosheets containing aluminum oxyanions as the active support for raman testing, exceptionally sensitive but non-selective enhancement is achieved. In addition, the SERS effect of the modified titanium carbide is applicable to a variety of different analyte molecules, including organic dyes and trace amounts of harmful compounds. Ti of the invention₃C₂The detection limit of the Raman detection substrate prepared by the nano-sheets is 8 multiplied by 10^‑12Lower than the blank substrate, Ti, which produced the same signal intensity₃C₂OH/F chip or even noble metal substrate.

Description

Application of two-dimensional titanium carbide nanosheet in Raman test, Raman test substrate and preparation method thereof

Technical Field

The invention belongs to a Raman test technology, and particularly relates to application of a two-dimensional titanium carbide nanosheet in Raman testing, a Raman test substrate and a preparation method thereof.

Background

Surface Enhanced Raman Spectroscopy (SERS) is a powerful noninvasive vibrational spectroscopy technique that provides orders of magnitude enhancement of raman intensity, combines the specificity of raman features with high sensitivity, and allows the determination of chemical properties and structural information even for orders of magnitude less molecules. The success of SERS depends largely on the selection of a suitable enhancing substrate. Existing SERS-active substrates are composed primarily of two major types of materials, including Au, Ag, and Cu metals with rough surfaces, or nanostructured semiconductors with non-stoichiometric or amorphous characteristics. A key contribution of metal substrates is electromagnetic enhancement, where highly curved or gapped regions of the metal generate so-called hot spots or greatly enhance the local electromagnetic field by local surface plasmon resonance effects. In addition, chemical enhancement based on charge transfer between the molecule and the substrate also contributes to raman enhancement, involving an increase in the polarizability of the molecule and the raman scattering cross-section. SERS signals on the noble metal SERS substrate mainly come from the former electromagnetic mechanism, have the advantage of high enhancement and result in low detection limit; however, in addition to high cost considerations, enhancement is often heterogeneous and requires complex nanostructures. Most semiconductor substrates employ chemical enhancement mechanisms to achieve raman signal enhancement, which can effectively accommodate non-uniformity and cost issues, but some key performance parameters remain far from satisfactory. SERS signals based on chemical mechanisms are generally low, and the detection limit of most semiconductor substrates is 10^-3-10^-7M, is higher than the detection limit obtained with complex noble metal substrates, which limits the application of the material as a sensitive surface enhanced substrate. In addition, both cases suffer from the same problem of high sensitivity being detectable only at selected points, and hence a severe lack of detection uniformity and reproducibility. Therefore, the SERS sensitivity and uniformity of the semiconductor enhancing material must be further improved.

Another important issue with SERS analysis relates to the possibility of preserving the native structure and conformation of the analyte molecules in the SERS measurement. The interaction of analyte molecules with many conventional semiconductor-or noble-metal-based SERS substrate surfaces often perturbs their native structure or conformation. For example, free radical porphyrins on Ag substrates can be metallized by binding Ag atoms from the surface into porphyrin macrocycles, which certainly leads to disappearance of certain molecular fingerprint information and interference of stray signals. Sometimes even when the analyte structure is not altered by surface interactions, the raman spectrum is altered because the symmetry of the molecular and/or electronic arrangement may be disturbed or even destroyed at the surface, in which case the intensity of a particular raman band depends on the orientation of the molecules with respect to the substrate surface. One solution to this problem is to introduce a molecular spacer between the substrate surface and the analyte, thereby maintaining the appropriate distance while still allowing for the SERS effect. However, it is well known that the SERS effect is very sensitive to distance and sometimes the interaction between the analyte and the enhancing matrix is somewhat diminished, leading to reduced performance. Therefore, it is desirable, but extremely challenging, to design a reinforced substrate with molecules on the substrate surface and maintaining the intrinsic structure and orientation of the molecules.

Disclosure of Invention

The invention discloses Ti with surface in-situ modified aluminum (Al) oxyanion₃C₂As a raman test substrate, high sensitivity is obtained, molecular detection can be performed at very dilute concentrations up to pM levels while retaining the intrinsic fingerprint information characteristics, the substrate has strong but non-selective interaction with the analyte; the Raman substrate can detect a series of analyte molecules including organic dyes, Methylene Blue (MB), rhodamine 6G (R6G), Brilliant Green (BG), Methyl Green (MG), Crystal Violet (CV), Nile Blue (NB), harmful chemical substances in daily life, Sudan III, Phenanthrene (PHEN), p-aminobenzoic acid (PABA), 4-mercaptobenzoic acid (4-MBA) and the like at low concentration; the new Raman substrate disclosed by the invention successfully highlights the SERS effect of the surface pair for the first timeThe importance of the SERS is high, and a new strategy for adjusting the SERS behavior is provided.

The invention adopts the following technical scheme:

Ti₃C₂nanosheets, the method of preparation thereof comprising the step of reacting Ti₃AlC₂Adding the powder into an organic alkali aqueous solution, stirring and centrifuging to obtain Ti₃C₂Nanosheets.

A Raman test substrate is prepared by mixing Ti₃AlC₂Adding the powder into an organic alkali aqueous solution, stirring and centrifuging to obtain Ti₃C₂Nanosheets; then adding Ti₃C₂Re-dispersing the nano-sheets in water to form a colloidal solution; and then dripping the colloidal solution on a nonmetal carrier to obtain the Raman test substrate.

In the invention, the organic alkali is tetramethylammonium hydroxide, and the invention creatively does not adopt the conventional hydrofluoric acid treatment Ti of the prior art₃AlC₂Powder, tetramethylammonium hydroxide, first disclosed by reaction with OH^-Reactive etching of the intermediate layer Al, TMA⁺Inserted into the interlayer space, the product Al (OH)₄In-situ modifying the surface Ti layer to obtain Ti₃C₂Nanosheet and conventional Ti₃C₂The nanosheets being different so as to obtain different technical effects, in particular Ti of the present invention₃C₂The nanosheets confirmed Al (OH) on their surface by one-dimensional Fourier analysis₄In the same concentration of Ti₃C₂-Al(OH)₄The Raman signal intensity of the MB molecules on the substrate is about Ti₃C₂3 times the Raman signal intensity of the MB molecules on the-OH/F substrate and the noble metal Au substrate, Ti₃C₂-Al(OH)₄The sheet substrate showed a significant SERS effect. The invention thus discloses the above Ti₃C₂The application of the nano sheet in Raman testing or preparation of a Raman testing substrate, in particular surface enhanced Raman testing.

In the invention, the concentration of the tetramethylammonium hydroxide aqueous solution is 20-30 wt%, preferably 25 wt%; stirring for 20-30 hours at room temperature, preferably for 24 hours; the rotation speed of the centrifugation is 3000-4000 rpm, preferably 3500 rpm.

In the present invention, Ti₃AlC₂The dosage ratio of the powder and the tetramethylammonium hydroxide is 1g to 10 ml.

In the present invention, Ti is added₃C₂Dispersing the nano sheets in water to form a colloidal solution; and dripping the colloidal solution on a nonmetal carrier, naturally spreading the colloidal solution, and naturally airing the colloidal solution in the air to obtain the Raman test substrate. The non-metal carrier is preferably a silicon substrate.

Ti of the invention₃C₂The detection limit of the Raman detection substrate prepared by the nano-sheets is 8 multiplied by 10^-12(ii) a Same signal intensity in blank substrate, Ti₃C₂Higher concentrations of-OH/F flakes and even of noble metals, respectively, of 1.9X 10^-5、8.4×10^-8And 8.0X 10^-8M can be observed; as known from the prior publications, the Ti of the present invention₃C₂-Al(OH)₄The substrate is the lowest detection limit of the semiconductor substrate. Surprisingly, despite Ti₃C₂-Al(OH)₄The sheet material shows a stronger reinforcing effect, but itI _ring/I _skeletonThe data overlap with that of a blank substrate without any reinforcing media and special interactions, the molecules adopt a flat configuration before reaching the critical point of complete monolayer tiling, and therefore, Ti₃C₂-Al(OH)₄The analyte conformation on the substrate of the sheet is similar to that on the bare blank substrate: lying in the dilution region and lying on the side in the high concentration region; in the whole concentration range, Ti₃C₂of-OH/F sheets and Au substratesI _ring/I _skeletonThe values are lower, indicating that the molecules lie at a greater angle regardless of the concentration or amount of analyte molecules.

In the present invention, the raman test refers to a surface enhanced raman test; surface enhanced raman testing (SERS) uses semiconductor non-noble metal substrates because of their advantages of low cost and abundant candidate selection, but its application is severely hampered by the difficulty of achieving satisfactory detection sensitivity, poor uniformity and undesirable vibrational signals by altering the orientation and/or polarizability of the probe molecules. The invention realizes the enhancement of abnormal sensitivity but non-selectivity by using the in-situ modified titanium carbide nanosheet containing the aluminum oxyanion as an active carrier for Raman measurement. The analyte molecules assume a conformation similar to that they adopt on the blank substrate, while interacting tightly with the aluminooxyanion surface, which leads to rare, highly sensitive but non-selectively enhanced observations, with detection limits approaching pM levels. Since the substrate surface roughness is on the nanometer scale, excellent uniformity is obtained with a relative standard deviation of less than 4.3%. In addition, the SERS effect of the modified titanium carbide is applicable to a variety of different analyte molecules, including organic dyes and trace amounts of harmful compounds. The invention successfully proves the feasibility of improving the SERS effect by regulating and controlling the surface, and introduces a new window for the application of the two-dimensional material in SERS.

Drawings

Fig. 1 is a schematic of nanosheet synthesis;

FIG. 2 is a comparative XRD pattern;

FIG. 3 is a one-dimensional Fourier analysis plot along the stacking direction of sample structures reacted in TMAOH;

FIG. 4 is a structural characterization of the nanoplatelets;

FIG. 5 is a Raman test chart;

FIG. 6 shows the assignment of MB major Raman peak positions on a blank substrate;

FIG. 7 shows different concentrations of MB molecules on a blank substrate, Ti₃C₂-Al(OH)₄Nanosheets, Ti₃C₂-raman signal enhancement of substrates of OH/F nanoplates and noble metals Au; the detection limits are respectively 2 multiplied by 10^-5、8×10^-12、9×10^-8And 8X 10^-8M；

FIG. 8 is a comparison of Raman intensities; in a blank substrate, Ti₃C₂-Al(OH)₄Reinforcing substrate, Ti₃C₂-OH/F reinforcing substrate and (a) on a noble metal Au substrateI _ring、I _skeletonAnd (b)I _ring/I _skeletonAs a function of concentration, (c) the MB molecule is in Ti₃C₂-Al(OH)₄And Ti₃C₂Schematic representation of orientation on OH/F flakes, (d) MB-modified Ti₃C₂-Al(OH)₄Nanosheet and Ti₃C₂Comparison of the UV-Vis absorption spectra of the-OH/F nanoplates with pure MB, all measurements being carried out in solution;

FIG. 9 is a MB adsorption isotherm;

FIG. 10 is a graph showing the estimation of the critical concentration of MB molecules on the substrate to achieve monolayer tiling;

FIG. 11 shows MB molecules at Ti₃C₂-Al(OH)₄And Ti₃C₂-a schematic of the structure on OH/F nanoplates;

FIG. 12 shows different doses of H₂O₂Oxidized Ti₃C₂-Al(OH)₄(ii) XRD and (b) XPS spectra of the flakes, (c) corresponding MB Raman spectra of the sample;

FIG. 13 shows the SERS effect of other analyte molecules and some harmful chemicals in daily life, including (a) R6G, (b) BG, (c) Sudan III and (d) PABA;

FIG. 14 shows Ti using the present invention₃C₂-Al(OH)₄The sheet enhanced substrate detects SERS spectra of other dyes and harmful molecules, including (a) Crystal Violet (CV), (b) Methyl Green (MG), (c) Nile Blue (NB), (d) Phenanthroline (PHEN) and (e) 4-mercaptobenzoic acid (4-MBA);

FIG. 15 shows a blank substrate, Ti₃C₂-Al(OH)₄Sheet and Ti₃C₂-the OH/F sheet enhances the raman spectrum of R6G on the substrate;

FIG. 16 shows Ti according to the present invention₃C₂-Al(OH)₄Enhancing the height uniformity and repeatability of the substrate;

FIG. 17 is (a) Ti with deposited analyte molecules₃C₂-Al(OH)₄Representative images of the thin plate enhanced substrate and raman collection at 10 randomly selected points on the substrate surface, (b) no significant variation and high uniformity of spectra collected.

Detailed Description

Ti of the invention₃C₂The preparation method of the nano-sheet comprises the following steps of mixing Ti₃AlC₂Adding the powder into an organic alkali aqueous solution, stirring and centrifuging to obtain Ti₃C₂Nanosheets.

The preparation method of the Raman test substrate comprises the following steps of mixing Ti₃AlC₂Adding the powder into an organic alkali aqueous solution, stirring and centrifuging to obtain Ti₃C₂Nanosheets; then adding Ti₃C₂Dispersing the nano sheets in water to form a colloidal solution; and then dripping the colloidal solution on a non-metal carrier to obtain the Raman test substrate. Especially surface enhanced raman substrates.

The surface enhanced Raman test method comprises the steps of placing a Raman substrate loaded with an analyte on an objective table, (1) setting instrument parameters, setting a filter (laser attenuation) to be D0.3 (50%), setting a confocal hole and a slit to be 400 and 100 mu m respectively, setting a grating to be 1800, selecting an objective lens to be multiplied by 50, and setting real-time measurement time to be 5 s; (2) set spectrometer 520.7 cm^-1Correcting the Si peak until the Si peak position is near 520.7; (3) setting a measurement range; (4) focusing the sample by white light to make the surface of the sample image most clearly, then turning off the white light, turning on the laser, clicking the test button, and starting the test. The test process needs to be kept quiet, and the light is turned off to eliminate the influence of other external factors.

Example one

At room temperature, add 1g of Ti₃AlC₂The powder (Beijing Fosmann) was stirred in 10 ml of aqueous tetramethylammonium hydroxide (TMAOH, 25 wt%) at 350 rpm for 24 hours, then washed by centrifugation and dispersed in H₂Shaking in O and centrifuging at 3500 rpm to obtain Ti with concentration of 1.3 mg/ml₃C₂-Al(OH)₄Colloidal solution of nanoplatelets, used in example two.

For Ti having a passivation layer on the surface₃AlC₂The powder was treated with very dilute HF (10 wt% HF) for 5 minutes to remove the surface passivation oxide layer before adding the aqueous tetramethylammonium hydroxide solution.

Comparative example 1

1g of Ti at room temperature₃AlC₂The powder (Beijing Fosman Co.) was stirred in 10 ml of 50 wt% HF aqueous solution at 350 rpm for 24 hours to remove the intermediate Al layer, then thoroughly washed with deionized water to remove excess HF, the wet precipitate was reacted in DMSO for 24 hours under magnetic stirring, thereby inserting a large amount of the guest in the interlayer, and finally the inserted sample (precipitate) was redispersed in ultrapure water and subjected to ultrasonic treatment, and then centrifuged at 3500 rpm to obtain Ti at a concentration of 0.4 mg/ml₃C₂-OH/F colloidal solution, i.e. conventional Ti₃C₂OH/F flakes for comparative example No. two.

FIG. 1 is a schematic diagram of the preparation of the two types of nanosheets, and FIG. 2 is the original Ti₃AlC₂The XRD pattern of the crystals is compared to that of the sample reacted in HF followed by insertion of DMSO and in TMAOH, and the small angle shift of the substrate reflection peak is due to the reduction of interlayer forces and concomitant guest intercalation.

The invention creatively prepares Ti₃C₂-Al(OH)₄Nanosheet, confirmed by one-dimensional Fourier analysis (FIG. 3) of Al (OH) on the surface thereof₄As a result, Al (OH) is present on the surface of the nanosheet₄A better fit is possible.

In the structural characterization of the sheet of FIG. 4, a and b are each exfoliated Ti₃C₂-Al(OH)₄And Ti₃C₂STEM image of-OH/F plate, c is the same as the parent Ti₃AlC₂Compared with¹³C NMR spectrum, d is Ti₃C₂-Al(OH)₄In-plane XRD pattern of the sheet, and e is Ti₃C₂-Al(OH)₄And f is Ti₃C₂AFM images of a monolayer of-OH/F flakes deposited on a silicon substrate. The Scanning Transmission Electron Microscope (STEM) images shown in fig. 4a and 4b show the different surface structures between the two types of lamellae, respectively, and the inset is a schematic structural view. For conventional Ti₃C₂Only three Ti layers were observed for the-OH/F sheets, whereas for the Ti of the present invention₃C₂-Al(OH)₄Sheet, two additional arrays of atoms can be clearly observed between the Ti3 layers due to the surface covering Al oxyanions; their differences in the local coordination environment were detected by Nuclear Magnetic Resonance (NMR) spectroscopy. As shown in fig. 4c, the solid state is collected¹³The C NMR spectrum showed that Ti₃AlC₂The parent crystal showed strong resonance at 550 ppm and a weaker shouldering signal at 517 ppm, which may be related to the residual TiC in the synthesis, and when the structure was delaminated into a monolayer, the Ti-Al bond was broken, resulting in Ti₃C₂-OH/F and-Al (OH)₄The chemical shifts of the plates were reduced to 387 and 411ppm, respectively. Ti of the invention₃C₂-Al(OH)₄The small shift change exhibited by the sheet also demonstrates that its different chemical environment has additional Al coordination.

Example two

Ti of example one₃C₂-Al(OH)₄The colloidal solution of the sheet was dropped on a silicon substrate (Zhongjing Ke apparatus Co.), naturally spread and then naturally dried in the air to obtain an SERS active substrate. Preparation of a monolayer of Ti on a silicon substrate₃C₂-Al(OH)₄Slicing; in-plane X-ray diffraction characterization was performed and the pattern can be assigned as a hexagonal unit cell with good agreement with the parent structure in the ab-plane (fig. 4 d), demonstrating good maintenance of the host structure.

Comparative example No. two

Ti of comparative example I₃C₂dropping-OH/F colloidal solution on silicon substrate (mesoscope), spreading naturally, and air drying to obtain Ti₃C₂-an OH/F active substrate.

In order to directly observe the thickness of the stripping sheet, an Atomic Force Microscope (AFM) test is carried out, the nano sheet is spread on the substrate in a single-layer or multi-layer covering mode, the thickness of the nano sheet layer is only 1-2 nm, therefore, the substrate has extremely uniform nano-scale surface roughness, possible generated 'hot spots' are eliminated, and a foundation is provided for high-uniformity Raman signal acquisition.

EXAMPLE III

For raman measurements, a concentration of analyte molecules is dropped onto the surface of a raman test substrate, followed by collection of raman spectra on a high resolution confocal raman spectrometer (LabRAM HR-800). An excitation wavelength of 632.8nm was generated with a He-Ne laser with a power of 0.7 mW; ar with power of 0.25 mW⁺The laser generates a wavelength of 514.5 nm. All acquisitions used a 50-fold microscope objective with an average spot size of 1 μm in diameter for the laser beam. The detector is a CCD detector with thermoelectric cooling to-70 ℃. Note that: all raman spectra were collected using a 632.8nm wavelength as excitation source, except that in the case of R6G a wavelength of 514.5 nm was used. All raman spectra have the peaks of the Si substrate subtracted. For each data point, 20 sets of spectra were collected, each set having an integration time of 5 s, and averaged for final analysis to eliminate intensity fluctuations.

The invention creatively provides a new Raman test substrate for Raman testing of compounds, the related compound solution and the specific Raman test method are conventional technologies, namely, the substrate of the existing Raman test is replaced by the Raman test substrate prepared by the invention creatively, the rest is unchanged, and the technical effects that the detection concentration is greatly lower than the existing lowest detection concentration, the molecular conformation is maintained unchanged and the fluorescence is inhibited can be realized.

The diluted solution containing the analyte molecules was dropped directly onto the raman test substrate, and the surface-modified Ti was examined using a series of probe molecules₃C₂Raman-enhanced behavior of the nanoplatelets. FIG. 5 shows a Raman test chart of (a) Ti₃C₂Schematic representation of a sample as a raman analyte enhancing substrate, (b) MB (10) deposited on a blank silicon substrate^-3M) Raman spectrum, (c) at Ti₃C₂-Al(OH)₄Substrate (SERS active substrate of example two), Ti₃C₂-OH/F substrate (comparative example II Ti)₃C₂-OH/F active substrate), precious metal reinforced substrate (commercially available) and collected MB (10) on blank silicon substrate^-5M) Raman spectra of aqueous solutions. As a typical example, a blank substrate is first collected under a laser having a wavelength of 632.8nmMB（10^-3M) to determine the characteristic spectrum of Methylene Blue (MB) at wavenumber of 300-1800 cm as shown in FIG. 5b^-1The range exhibits a strong signal. 446cm can be used^-1And 1622 cm^-1The strong characteristic raman bands at (a) are due to framework C-N-C deformation and C-C stretching vibration of the fused aromatic rings, respectively (inset in fig. 5b and fig. 6). In a typical raman observation test, the signal weakens when the concentration of analyte molecules is diluted, and when the concentration is 2 × 10^-5M, no clear signal is seen (fig. 7). The molecules were deposited on a commercial Au substrate, or surface Al (OH), as shown by the Raman spectrum of FIG. 5c₄or-OH/F Ti₃C₂Strength is significantly enhanced with a lamellar substrate, at which concentration Ti is present₃C₂-Al(OH)₄The Raman signal intensity of the MB molecules on the substrate is about Ti₃C₂3 times the Raman signal intensity of the MB molecules on the-OH/F substrate and the noble metal Au substrate, Ti₃C₂-Al(OH)₄The sheet substrate showed a significant SERS effect.

The extremely low detection limit also reflects Ti₃C₂-Al(OH)₄Excellent SERS performance. For Ti₃C₂-OH/F substrate and noble metal Au substrate at 9X 10^-8And 8X 10^-8Almost no Raman signal was seen at M, while at Ti₃C₂-Al(OH)₄On the substrate, even when MB was further diluted by 4 orders of magnitude, the concentration was close to pM level, 8.0X 10^-12The raman signal is still clear at M (fig. 8a and 7). Same signal intensity in blank substrate, Ti₃C₂Higher concentrations of-OH/F flakes and even of noble metals, respectively, of 1.9X 10^-5、8.4×10^-8And 8.0X 10^-8M is observed. As known from the prior publications, the Ti of the present invention₃C₂-Al(OH)₄The plate is the lowest detection limit in the semiconductor substrate.

The invention is based on Ti₃C₂-Al(OH)₄The SERS-active substrate of (a) eliminates the possible effect of medium adsorption capacity on enhanced performance (fig. 9). When the intensity variations at different concentrations are examined,note that the intensity showed a tendency to gradually increase in the low concentration region until it became 1.0X 10^-6A plateau is reached at M, which is related to the amount of analyte molecules involved, the inflection point is considered to be monolayer tiling, and the critical concentration for achieving monolayer tiling of MB analyte is estimated by geometric considerations, which yields a derivative of 1.8 × 10^-6M (FIG. 10). Beyond the critical point of monolayer tiling, the increase in raman intensity is very limited as the analyte concentration increases.

By comparison, the bending of the corresponding C-N-C skeleton (446 cm)^-1) And aromatic ring stretching vibration (1626 cm)^-1) Intensity ratio of signals to confirm conformation of MB analyte, i.e.I _ring/I _skeletonFurther understanding this unusual trend of change; as shown in fig. 8 b. The conformational selection of the probe molecule usually occurs due to strong interactions of the molecule with the enhancing substrate, which causes spectral changes and sometimes difficulties in the molecular assay. Surprisingly, despite Ti₃C₂-Al(OH)₄The sheet material shows a stronger reinforcing effect, but itI _ring/I _skeletonThe data is overlaid with the data of a blank substrate without any enhancement media and special interaction. On such a blank substrate, the molecules adopt a flat configuration before reaching, at low concentrations, the critical point of complete monolayer tiling, which is approximately 1.8 × 10^-6M (depending on geometric considerations) (see above), at higher concentrations, the electrostatic repulsive forces between the molecules may gradually drive the molecules to adopt a lateral configuration and aggregate. In the experiment, the raman signal on the blank silicon substrate was not detectable in the dispersed lying state, only if the concentration of the analyte was greater than 2.0 × 10 in the lateral aggregation state^-5M is observable. Ti₃C₂-Al(OH)₄The data for the substrate and the data for the blank substrate overlap indicating that the analyte molecules adopt a similar conformation by interacting with the substrate via the lone pair of electrons of the N atom. In the dilution zone, higher is foundI _ring/I _skeletonIndicating that its lay-flat configuration and the aromatic ring interact more strongly with the substrate. Binding signal strength hereA linear increase in range may reasonably infer that the analyte MB molecules adopt a lying conformation before reaching monolayer laying. Thus, Ti₃C₂-Al(OH)₄The analyte conformation on the substrate of the sheet is similar to that on the bare blank substrate: lying flat in the dilution zone and lying on the side in the high concentration zone. In the whole concentration range, Ti₃C₂of-OH/F sheets and Au substratesI _ring/I _skeletonThe values were lower, indicating that the molecules lie at a larger angle regardless of the concentration or amount of analyte molecules (fig. 8 c).

This conformational difference has been confirmed by density functional theory calculations, which are mainly due to the interaction between aromatic rings and the groups on the surface of the reinforcing substrate. At Ti₃C₂-OH/F flakes on the surface, analyte MB molecules preferentially adopt the lateral configuration, interacting with the substrate with C-N bonds; in contrast, in Ti₃C₂-Al(OH)₄On the sheet, the surface is mainly OH, which can interact tightly with the ring N and S atoms, and thus can form a relatively flat conformation.

Table 1 summarizes the positional shift of the characteristic bands relative to the spectrum collected on the blank substrate, except for approximately 1428 cm, attributed to C-N asymmetric stretching vibration^-1Out of the band of the raman spectrum, there is no significant change in band position for almost all of the raman bands. This strongly suggests that the MB molecules are attached to the enhancement substrate by C — N bonds, which is highly consistent with molecular conformation simulation results and illustrates the significant enhancement effect of the sheet-like substrate. Notably, Ti₃C₂-Al(OH)₄The C-N bond length variation on the sheet is almost Ti₃C₂3 times the change in C-N bond length on-OH/F sheets (FIG. 11, Table 2), indicating analyte and Ti₃C₂-Al(OH)₄More efficient charge transfer between the sheets, comparing bond length and Bader charge changes for the two types of sheets, indicates that the change in C-N on the ring is most pronounced, while Ti₃C₂-Al(OH)₄The change in bond length and the total charge transfer amount of the sheet are almost all Ti₃C₂-3 times OH/F flakes. Furthermore, the UV-visible absorption as in FIG. 8d absorbs lightSpectrum shows, deposit on Ti₃C₂-Al(OH)₄The spectral absorption peak position of the analyte MB molecule on the chip was red-shifted from 654 nm to 687 nm with Ti₃C₂The very slight shift observed for the-OH/F sheet forms a sharp contrast. Since large shifts in the absorption peaks generally indicate the formation of a strong and complex, essentially efficient charge transfer process, accounting for the analytes MB and Ti₃C₂Efficiency of charge transfer between Ti and Ti₃C₂-Al(OH)₄Sheet material greatly exceeds Ti₃C₂-OH/F sheet; ti₃C₂-Al(OH)₄The amount of charge transfer between the flake and MB was estimated to be 1.21 electrons by Bade theory analysis and to be smaller than that of Ti₃C₂The amount of charge transfer (0.86 electrons) between the-OH/F flakes and the MB is much larger. Ti₃C₂-Al(OH)₄The effective charge transfer on the sheet is due to the lay-flat configuration which facilitates distance dependent charge transfer and therefore detection even at very limited electron counts. In addition, since the flat surface with uniform charge is gradually oxidatively destroyed, the strengthening effect becomes weaker, and Al (OH) is proved₄Criticality of end capping, see FIG. 12, ratio representing sheet to H₂O₂Only in keeping with Ti₃C₂The inherent structure of the sheet is simultaneously controlled to modify its surface, it is clear that the signal intensity follows H₂O₂The dose is increased and decreased, confirming the important role of the aluminum oxyanion group as a surface ligand for raman enhancement. With other semiconductor materials (e.g. TiO)₂，MoO₂And W₁₈O₁₉) Compared with the SERS substrate, the SERS substrate has higher detection sensitivity, and simultaneously retains the advantages of molecular characteristic fingerprint information, while Raman signals of other semiconductors are mainly based on chemical enhancement and are inconsistent with the inherent characteristic spectrogram. Furthermore, the linear change in SERS signal corresponding to the analyte concentration can be quantitatively detected in the concentration range from pM to μ M.

TABLE 1 characteristic Raman signals of MB and their presence at Ti₃C₂-Al(OH)₄And-relative displacement (SERS displacement minus Raman displacement) on the enhanced substrate of the OH/F plate

TABLE 2 bond length and Bader Charge variation

Ti of the invention₃C₂-Al(OH)₄The enhanced substrate of the sheet shows very wide applicability, the MB solution is replaced by other molecular solution, the Raman detection is carried out by adopting the same method, and other trace dye molecules, such as rhodamine 6G (R6G), Crystal Violet (CV), Brilliant Green (BG), Nile Blue (NB) and Methyl Green (MG), can be detected even at extremely low concentration; more importantly, the substrate also has good responsiveness to various common harmful chemicals in daily products. For example, it is possible to use 10 on this novel SERS substrate^-5The level of M was effectively detected for sudan III, Phenanthroline (PHEN), para aminobenzoic acid (PABA) and 4-mercaptobenzoic acid (4-MBA), see fig. 13 and 14, indicating its excellent applicability and versatility as a SERS substrate material. These molecules also retain the native conformation of the analyte molecule.

Ti of the invention₃C₂Another interesting observation of the thin plate as a SERS substrate is the strong fluorescence quenching effect, FIG. 15 is the blank substrate, Ti₃C₂-Al(OH)₄Sheet and Ti₃C₂-the OH/F sheet enhances the raman spectrum of R6G on the substrate; the fluorescent background was separately Ti compared with the blank substrate₃C₂-Al(OH)₄Sheet and Ti₃C₂the-OH/F plate enhanced the substrate quenching 37 and 10 fold. It is known that organic fluorescent dye R6G generally produces a large fluorescent background, thereby overwhelming the SERS spectral signal of R6G dye. However, Ti₃C₂The spectrum of R6G on the plate showed a very large fluorescence suppression effect with an almost flat background, especiallyIt is Ti₃C₂-Al(OH)₄A substrate. This is often done in graphene, MoO₂，WTe₂And the high fluorescence observed on other semiconductor substrates is quite different. The strong fluorescence quenching effect is probably similar to that of R6G and Ti₃C₂The effective charge transfer between the sheets is relevant. In particular, the molecule R6G is in Ti₃C₂-Al(OH)₄The tiling of the single layer on the plate enables sufficient contact and further causes a reduction in the fluorescence cross section.

Most reinforcing matrices are composed of microparticles, and agglomeration inevitably occurs, some of which depend on the formation of hot spots, leading to problems of low uniformity and reproducibility. In the case of the present invention, since the roughness of the substrate is low, high uniformity is obtained. FIG. 16 shows Ti according to the present invention₃C₂-Al(OH)₄Enhancing the height uniformity and repeatability of the substrate. (a) SERS spectra were collected at 100 points by line scanning and collected at 1 μm step size 1625 cm from spot (b)^-1The intensity of the vibration band changes. (c) The SERS spectra of MBs collected from 500 randomly selected positions on 50 batches of substrates, the overlapping signals form a narrow shaded area. Illustration is shown: the intensity variation from batch to batch; FIG. 16a shows MB at Ti₃C₂-Al(OH)₄SERS spectra on a substrate. By line scanning 100 different spots over an area of width 100 μm, the obtained raman signals are very similar. Characteristic peak 1625 cm^-1The Relative Standard Deviation (RSD) of (d) was 4.3% (fig. 16 b), demonstrating a higher uniformity. In addition, batch-to-batch repeatability was examined by making 50 batches of enhanced substrates and collecting the SERS spectra of MBs from 500 randomly selected points (10 points per batch) as shown in fig. 16c and 17. These spectra form narrow shaded regions, indicating that the SERS signal fluctuations for substrates between batches are relatively small. Calculated at 1625 cm^-1RSD of 2.42%, much lower than electrochromic SERS substrate (6.79%), hydrogen treated W₁₈O₄₉Sample (30.2%) and commercial Klarite plaque (29.0%). These results confirm the Ti of the present invention₃C₂-Al(OH)₄The reinforced substrate has higher batch reproducibility. Apparently, Ti₃C₂-Al(OH)₄The sheet substrate has 4 advantages as a raman enhancing substrate, including (1) high enhancement effect and low detection limit, (2) ability to retain a range of molecular fingerprint information, (3) broad applicability, and (4) good fluorescence quenching efficiency, which makes the material unique in SERS substrates.

In summary, Ti is modified by aluminum oxyanions₃C₂On the surface of the nano sheet, the invention firstly obtains high SERS sensitivity, the detection limit is reduced to pM level, and simultaneously, the unconventional nonselective enhancement of the inherent Raman signal is realized. In addition, high uniformity and reproducibility of SERS signals may be achieved due to low roughness below the nanoscale. Detailed experimental studies have demonstrated that these advantages are related to the ultra-thin thickness of the two-dimensional sheet and its adjustable surface characteristics. Due to the close interaction of the analyte with the aluminum oxyanion, the analyte may adopt a configuration similar to the blank substrate, with the C — N lying flat against the substrate with a slight twist. An unusual combination of high sensitivity and signal enhancement is achieved while also exhibiting no obvious choice. The success of this work underscores the importance of surface features for SERS and opens new windows for the design of semiconductor SERS substrates.

Claims (10)

1.Ti₃C₂Nanosheets characterized by the Ti₃C₂The preparation method of the nano-sheet comprises the following steps of mixing Ti₃AlC₂Adding the powder into an organic alkali aqueous solution, stirring and centrifuging to obtain Ti₃C₂Nanosheets.

2. The Ti of claim 1₃C₂Nanosheets characterized in that the organic base is tetramethylammonium hydroxide; the concentration of the organic alkaline water solution is 20-30 wt%; stirring for 20-30 hours at room temperature; the rotating speed of the centrifugation is 3000-4000 rpm.

3. A Raman test substrate is characterized in thatThe preparation method of the Raman test substrate comprises the following steps of mixing Ti₃AlC₂Adding the powder into an organic alkali aqueous solution, stirring and centrifuging to obtain Ti₃C₂Nanosheets; then adding Ti₃C₂Dispersing the nano sheets in water to form a colloidal solution; and then dripping the colloidal solution on a nonmetal carrier to obtain the Raman test substrate.

4. A raman test substrate according to claim 3 wherein the organic base is tetramethylammonium hydroxide; the concentration of the organic alkaline water solution is 20-30 wt%; stirring for 20-30 hours at room temperature; the rotating speed of the centrifugation is 3000-4000 rpm.

5. A raman test substrate according to claim 3, wherein Ti is incorporated₃C₂Dispersing the nano-sheets in water, and shaking to form a colloidal solution; and dripping the colloidal solution on a nonmetal carrier, naturally spreading the colloidal solution, and naturally airing the colloidal solution in the air to obtain the Raman test substrate.

6. A raman test substrate according to claim 3 wherein the non-metallic carrier is a silicon substrate.

7. Ti as set forth in claim 1₃C₂Use of nanoplatelets or a raman test substrate according to claim 3 for raman testing or for the preparation of a raman test substrate.

8. Use according to claim 7, wherein the Raman test is a surface enhanced Raman test.

9. Ti₃C₂A process for producing a nanosheet, comprising the step of subjecting Ti to₃AlC₂Adding the powder into an organic alkali aqueous solution, stirring and centrifuging to obtain Ti₃C₂Nanosheets.

10. Raman test baseA method for producing a substrate, characterized by comprising the step of subjecting Ti₃AlC₂Adding the powder into an organic alkali aqueous solution, stirring and centrifuging to obtain Ti₃C₂Nanosheets; then adding Ti₃C₂Dispersing the nano sheets in water to form a colloidal solution; and then dripping the colloidal solution on a nonmetal carrier to obtain the Raman test substrate.

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