CN111175284B - Preparation method of surface enhanced Raman substrate with layered micro/nano structure - Google Patents
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Abstract
The invention provides a preparation method of a surface enhanced Raman substrate with a layered micro/nano structure, which comprises the following steps: irradiating the surface of a silicon or silicon dioxide matrix by using nanosecond-femtosecond laser pulses to form a micro-pit array; cleaning and drying the substrate with the micro-pit array; and depositing to form a silver film in the micro-pit array area of the substrate. The MA-SERS with the layered micro/nano structure can be prepared by nanosecond-femtosecond laser treatment and magnetron sputtering; the prepared MA-SERS has good wettability, the contact angle is about 150 degrees, and the enhancement factor of surface enhanced Raman scattering can reach 1.3 multiplied by 107Sufficient to detect molecular levels; the preparation process is economical and efficient; the prepared MA-SERS can have wide application prospect in detection of biological drugs, pesticide residues and ultra-low concentration solution polluted by environment.
Description
Technical Field
The invention relates to the technical field of Raman detection of trace substances, in particular to a preparation method of a surface enhanced Raman substrate with a layered micro/nano structure.
Background
Generally, surface enhanced raman scattering with unique molecular vibrational fingerprints is used to identify analytes, providing an efficient non-invasive spectroscopic method for trace molecular detection in the biomedical/analytical field.
However, despite the great efforts devoted to developing various SERS substrates with graded micro/nanostructures to achieve sufficient sensitivity while maintaining adequate superhydrophobicity, combining superhydrophobicity with plasmonic nanostructures with hot spots by a controllable and efficient method remains a challenge.
Disclosure of Invention
The present invention aims to address at least one of the above-mentioned deficiencies of the prior art. For example, an object of the present invention is to provide a method for producing a surface-enhanced raman substrate capable of enhancing the raman detection effect.
In order to achieve the above object, the present invention provides a method for preparing a surface enhanced raman substrate having a layered micro/nano structure, the method comprising the steps of: irradiating the surface of a silicon substrate or a silicon dioxide substrate by using nanosecond-femtosecond laser pulses to form a micro-pit array, wherein the diameter of each micro-pit in the micro-pit array is 10-20 mu m, the distance between the outer edges of any two adjacent micro-pits is not more than 1/6 of the diameter, and the micro-pit array is provided with a plurality of nano convex points with the grain size of 30-100 nm; cleaning and drying the silicon substrate or the silicon dioxide substrate with the micro-pit array; and then, depositing and forming a silver film with the thickness of 40-300 nm in a silicon substrate or a silicon dioxide substrate area with a micro-pit array to obtain the surface enhanced Raman substrate with the layered micro/nano structure.
In an exemplary embodiment of the present invention, the number of pulses of the laser pulse of nanosecond to femtosecond corresponding to each dimple of the dimple array may be selected in a range of 8 to 40.
In an exemplary embodiment of the present invention, the power of the nanosecond-femtosecond laser pulse may be 3-10 mW.
In an exemplary embodiment of the invention, each micro pit in the micro pit array may have a diameter of 13 to 17 μm, and a distance between outer edges of any two adjacent micro pits is from zero to 1/10 of the diameter, and the nano bump sites have a particle size of 65 to 85 nm.
In an exemplary embodiment of the present invention, the silver thin film may have a thickness of 100 to 180 nm.
Compared with the prior art, the invention has the beneficial effects that: the MA-SERS with the layered micro/nano structure can be prepared by femtosecond laser treatment and magnetron sputtering; the prepared MA-SERS has good wettability, the contact angle is about 150 degrees, and the enhancement factor of surface enhanced Raman scattering can reach 1.3 multiplied by 107And even higher, sufficient to detect molecular levels; the preparation process is economical and efficient; the prepared MA-SERS can have wide application prospect in detection of biological drugs, pesticide residues and ultra-low concentration solution polluted by environment.
Drawings
FIG. 1A shows a schematic of a preparation and detection process according to an exemplary embodiment of the present invention;
FIG. 1B shows a schematic of two stages of silver film deposition in an exemplary embodiment of the invention;
fig. 1C shows an SEM image of a surface enhanced raman substrate with layered micro/nanostructures made according to an exemplary embodiment of the present invention, wherein II and IV correspond to type 1, III and V correspond to type 2;
FIG. 1D shows a diameter statistical schematic of the silver nanoparticles of types 1 and 2 of FIG. 1C;
FIG. 1E shows the results of energy dispersive X-ray spectroscopy (EDX) in type 1 and type 2 of FIG. 1C;
fig. 2 a is a schematic diagram showing the concentration and amplification effect of the surface enhanced raman substrate with layered micro/nano structures on the analyte prepared by one exemplary embodiment of the present invention;
b in fig. 2 shows a schematic diagram of the enhancement principle of raman signal by the nanoparticles;
c and D in fig. 2 show the simulation results and the comparative analysis results of the raman spectra of the reagent of comparative example 1, comparative example 2 and one exemplary embodiment of the present invention, respectively;
a and B in FIG. 3 show the number of pulses or silver sputtering time, respectively, as a function of surface wettability in the method of the present invention;
FIGS. 3C and D are schematic diagrams showing the Raman enhancement effect of the surface-enhanced Raman substrate with a pulse number of 8 to 40, respectively, in the method of the present invention;
FIGS. 3E and F are schematic diagrams showing the Raman enhancement effect of the surface enhanced Raman substrate with a sputtering time of from 100s to 500s in the method of the present invention, respectively;
a in FIG. 4 shows the concentration of 1X 10-5The R6G molecule of M enhances the signal intensity at each of 10 points on type 1 and type 2 of the surface enhanced raman substrate of the present invention;
b and C in FIG. 4 show 611cm extracted from A in FIG. 4, respectively-1And 1650cm-1Data of peak processing;
a and B in fig. 5 show SEM images at different sites of comparative example 1 in C in fig. 2, respectively;
c in fig. 5 shows an SEM image of comparative example 2 in C in fig. 2;
fig. 6 shows a raman intensity plot of R6G powder on a glass substrate;
FIG. 7 is a schematic diagram showing sputtering time versus silver film thickness in an exemplary embodiment of the invention;
fig. 8 is a schematic diagram showing the electric field strength of a surface enhanced raman substrate of an exemplary embodiment of the present invention as a function of the range of diameters of silver nanospheres.
Detailed Description
Hereinafter, the method for preparing the surface enhanced raman substrate (also referred to as MA-SERS) having a layered micro/nano structure according to the present invention will be described in detail with reference to exemplary embodiments.
In one exemplary embodiment of the present invention, the method for preparing a surface enhanced raman substrate having a layered micro/nano structure may be achieved by the steps of:
(1) irradiating the surface of the substrate by nanosecond or femtosecond laser to form a micro-pit array
The layered ordered micro-pit array with the nano-scale structure is rapidly manufactured on a silicon wafer or a silicon dioxide wafer through nanosecond-femtosecond laser pulse dotting scanning. Each dimple of the dimple array has a diameter of 10 to 20 μm, and a distance between outer edges of any two adjacent dimples is not more than 1/6 of the diameter (e.g., an average value of diameters), the dimple array having a plurality of nano-bump points with a particle size of 30 to 100 nm.
Here, the nano-projection is also formed by irradiating a silicon wafer or a silicon dioxide wafer with a nanosecond to femtosecond laser pulse as the surface of the micro-pit array. Here, the number of pulses of the laser light corresponding to each dimple in the dimple array can be controlled to be selected in the range of 8 to 40, and further, can be controlled to be 10 to 25. The power of the laser pulse from nanosecond to femtosecond can be 3-10 mW. However, the present invention is not so limited as long as it is capable of producing an array of micro-pits of the desired size and topography.
For example, the substrate may be a silicon substrate or a silicon dioxide substrate with a high purity (e.g., not less than 99.9%), since a low purity may generate a strong background peak and thus affect the detection accuracy. The short pulse laser system for irradiating the substrate can generate laser pulses from nanosecond to femtosecond, thereby being beneficial to ensuring the processing effect, improving the processing precision, reducing the material damage and being not easy to generate cracks. And processing by using laser pulse with the power of 3-10 mW. The power range is favorable for forming a micro-pit array with high wettability, and excessively high power is easy to destroy the surface appearance and reduce the hydrophobicity. The laser pulse is Gaussian, the diameter of a light spot at a focusing plane can be 10-20 mu m, and the diameter range of the light spot is favorable for obtaining the expected diameter range of the micro-pits. The high-speed inching of the laser is accurately controlled by a scanning galvanometer suitable for a laser system, so that a dot matrix smaller than 5 multiplied by 5mm is generated on a Si substrate to adapt to trace liquid drop detection.
(2) Cleaning and drying
And (2) cleaning and drying the product obtained after the treatment in the step (1). For example, the cleaning may be performed by an organic cleaning liquid (e.g., ethanol) and ultra pure water, respectively, and the cleaning manner may be ultrasonic cleaning, however, the present invention is not limited thereto. The drying can be carried out in a vacuum oven, and the drying temperature can be 80-120 ℃, so that the rapid drying is facilitated. However, the present invention is not limited thereto.
(3) Depositing to form a silver film
And depositing and forming a 40-300 nm silver film (which can be called as a silver film for short) on the area with the micro-pit array of the substrate to obtain the surface enhanced Raman substrate with the layered micro/nano structure. That is, the silver thin film is deposited on the surface of the micro-pit array while being capable of wrapping the plurality of nano-projection points constituting a part of the surface of the micro-pit array. The inner core of the nano-particles formed by the silver-coated nano-convex points is silicon or the outer surface of silicon dioxide is a silver film, which can be called as silver nano-particles for short.
The silver film can be deposited by magnetron sputtering deposition. For example, the thickness of the silver film may be further 100 to 180 nm. The deposited silver film has low surface energy, can form a multilevel structure by matching with a micro-pit array, is favorable for realizing super-hydrophobicity, and the nano convex points coated by the silver film can provide good hot points.
The surface enhanced raman substrate having a layered micro/nano structure can be obtained by the production method of the present exemplary embodiment. The surface-enhanced Raman substrate is composed of a semiconductor substrate with a micro-pit array and a silver thin film deposited on the micro-pit array of the semiconductor substrate and having a thickness of 40-300 nm, wherein the micro-pit array is formed by irradiating the semiconductor substrate with a laser pulse from nanosecond to femtosecond, the diameter of each micro-pit in the micro-pit array is 10-20 μm, the distance between the outer edges of any two adjacent micro-pits is not more than 1/6 of the diameter, and the micro-pit array is provided with a plurality of nano convex points with the grain size of 40-90 nm.
Furthermore, the diameter of each micro pit in the micro pit array is 13-17 μm, the distance between the outer edges of any two adjacent micro pits is from zero to 1/10 of the diameter, and the particle size of the nano convex points is 65-85 nm. That is, the outer edges of any two adjacent micro-pits may be substantially tangent to each other, or the outer edges of any two adjacent micro-pits may be spaced apart and the distance between the outer edges may be no more than 1/10 the average diameter of the two adjacent micro-pits.
The method for preparing the surface-enhanced raman substrate having a layered micro/nano structure according to the exemplary embodiment of the present invention will be described in further detail with reference to the specific examples and the accompanying drawings.
Example 1
The substrate is a <100> n-type single crystal silicon having a resistivity of 1 to 5 [ omega ] cm. The sample was irradiated with a femtosecond laser system which generated laser pulses with a center wavelength of 800nm, a pulse width of 104fs, and a repetition frequency of 1 kHz. The laser beam was focused perpendicular to the surface of these samples using a power of 5 mW. The laser pulse is gaussian and the spot diameter at the focal plane is 14 μm. The high speed jog of the laser is precisely controlled by a scanning galvanometer adapted for use in a laser system to generate a 5 x 5mm lattice on a Si substrate. The number of pulses per point is chosen between 8 and 40.
Next, the substrate was subjected to ultrasonic cleaning for 10 minutes with ethanol and ultrapure water, respectively, in this order. Thereafter, the sample was dried in a vacuum oven at 100 ℃ for 20 minutes.
Subsequently, Ag films having thicknesses of 50.1nm, 92.7nm, 161.6nm, 225.5nm and 282.9nm, respectively, were deposited on the samples using a magnetron sputtering machine.
Results of the surface morphology and elements of the surface enhanced raman substrate having a layered micro/nano structure obtained in example 1, the enhancement principle and sensitivity, the influence of the number of pulses and the silver sputtering time on the enhancement effect, the uniformity of raman signal, and the like will be analyzed.
1. MA-SERS preparation and surface morphology and element analysis
Due to the explosiveness of femtosecond laser processing, femtosecond laser fabricated large-scale micro-pit arrays and nano-scale structures in a short time, followed by MA-SERS (fig. 1A) obtained by depositing a silver thin film. The process of silver film deposition can be divided into two stages (fig. 1B) including deposition of silver atoms on a sufficiently rough surface to form clusters of nanoparticles, and successive attachment of silver atoms, resulting in an increase in film thickness and particle size. Meanwhile, the deposition process can significantly reduce the surface energy to increase the hydrophobicity of the substrate without using low surface energy molecules such as fluorosilane, which contain relatively strong background Raman signals that may interfere with the Raman signals of the probe molecules. That is, the MA-SERS prepared by the invention is beneficial to reducing background Raman signals, thereby reducing interference.
SEM images (fig. 1C I) show MA-SERS with large-scale, highly uniform circular arrays of micro-pits, which benefit from high precision processing on the micron scale with femtosecond lasers. The further magnified SEM image of fig. 1C I (fig. 1C II, III) clearly shows that the micro-pits formed by the laser and the adjacent flat surface are defined as type 1 and type 2, respectively. As shown in the magnified SEM image (fig. 1C IV, V), a large amount of silver nanoparticles were deposited on the entire surface of the substrate. These abundant nanoparticles can provide satisfactory hot spots to improve the enhancement factor of surface enhanced raman scattering.
Statistical analysis of the diameter of silver nanoparticles on the MA-SERS surface was performed as shown in fig. 1D. The particle sizes of type 1 and type 2 are expressed as a normal distribution with average diameters of about 48 and 52nm, respectively. The particle size of type 2 is slightly larger than that of type 1 because the deposition of silver atoms is normally incident on type 2 and obliquely incident on type 1. Therefore, it will result in a higher deposition rate for type 2 than for type 2. Furthermore, it should be mentioned that elements attached to the substrate may generate strong background peaks, thereby affecting the accuracy of the detection. The energy dispersive X-ray spectroscopy (EDX) results (fig. 1E) indicate that the surface elements are only Si and Ag, which means that MA-SERS made by the present invention has few background elements and thus helps to improve detection sensitivity.
2. Enhancement principle and sensitivity of SERS substrate
During detection, analyte molecules are concentrated and concentrated at the hot spot, which in effect amplifies the density of the probe molecules (a in fig. 2). The final concentration of MA-SERS was about 0.79mm2Approximately 11.5 times as much as the hydrophilic substrate to achieve concentration amplification. According to the Cassie-Baxter model, the hydrophobicity 150 ° is attributed to the air gap between the water droplets and the Ag film with layered micro/nano structure. After the liquid is dried, the Local Surface Plasma Resonance (LSPRs) and the incidence of 514.5nm laser lightThe coupling of light results in an enhanced electric field (B in fig. 2) in the local region between adjacent nanoparticles.
To demonstrate that femtosecond laser treatment and magnetron sputter silver plating are necessary conditions for obtaining raman signal enhancement by MA-SERS, comparative analysis was performed using only a laser-treated silicon substrate (T-Si) (as comparative example 1), a silicon substrate (Ag/Si) deposited with only silver (Ag) and MA-SERS (Ag/T-Si) prepared according to the present invention (C, D in fig. 2).
The actual raman spectrum at D in fig. 2 and the simulation results at C in fig. 2 show that only Ag/T-Si substrates with strong electromagnetic field enhancement provide sufficient intensity signals. The substrate is simply modeled as a large number of particles randomly distributed on the substrate, as nanoparticles are a key factor in achieving surface-enhanced raman scattering. For comparative example 1, although the T-Si substrate in a super-hydrophilic state having a contact angle of 3 ° was covered with a large number of nanoparticles (a, B in fig. 5) and probe molecules entered the nanogap, hot spots provided by silicon were not satisfactory, and signal enhancement was difficult to achieve. For comparative example 2, the untreated silicon was too smooth to form discrete nanoparticles during magnetron sputtering, eventually forming a dense silver film on the single crystal silicon surface (C in fig. 5). Since the nanogap is too small, the probe molecules cannot enter, and thus the Ag/Si substrate having a contact angle of about 112 ° cannot achieve effective raman scattering enhancement of the probe molecules.
For MA-SERS produced by the present invention, Ag/T-Si substrates with contact angles of about 150 ° can generate a large number of electromagnetic hot spots of sufficient intensity, which not only helps to increase the concentration of the probe molecules, but also allows the probe molecules to enter the gap to obtain a satisfactory signal. That is, the MA-SERS prepared by the invention can generate enough hot spots for surface enhanced Raman scattering. The SERS Enhancement Factor (EF) of MA-SERS was experimentally estimated using the following formula:
wherein, ISERSAnd IRSRaman intensity on MA-SERS and glass substrate, respectively, and NSERSAnd NRSIs the number of probe molecules corresponding to these two substrates. To determine IRSAnd NRSThe rhodamine 6G (may be abbreviated as R6G) powder was pressed flat on a glass substrate, and a signal thereof was detected by a laser at a power of 85 μ W and a wavelength of 514.5 nm. Number of molecules (N)RS) Estimated to be 4.97X 1011，IRSThe results are shown in FIG. 6. Will have 4 μ L1 × 10-6M solution R6G was dropped on the substrate and air dried to confirm ISERSAnd NSERSAnd then detected with a 0.85 μ W laser. If the R6G solution is uniformly distributed after concentration, NSERSEstimated to be 9.63 × 106The calculated surface average EF can reach 1.3 multiplied by 107. According to calculation, the EF of the MA-SERS surface prepared by the method can be 1.2-1.5 multiplied by 107.
3. Effect of pulse number and silver sputtering time on enhancement Effect
The two parameters, pulse number and silver sputtering time, will affect the surface topography and thus the detection effect, and therefore additional analysis of these two parameters is needed to optimize signal enhancement. Considering that the microporous structure causes the transition from the hydrophobic surface (CA-112 °) to the hydrophobic surface (CA-150 °), the relationship between the number of pulses or the silver sputtering time and the surface wettability was first studied, respectively, and the results are shown as a and B in fig. 3, respectively. When the number of treatment pulses was increased from 8 to 40, no significant change in hydrophobicity was observed. In addition, no significant change in hydrophobicity was observed as the magnetron sputtering time was increased from 100 to 500s (corresponding to Ag film thicknesses from 50 to 283nm, as shown in FIG. 7). A possible explanation is that the structure of the micro crater array (which is the main contributor to wettability) does not change significantly within these two parameters. Satisfactory hydrophobicity helps to increase the detection limit by virtue of the aggregation of the probe molecule droplets. At a concentration of 10-5M R6G molecular solution was used as the analyte, and all the hydrophobic substrates with pulse number of 8 to 40 showed good Raman enhancement (C in FIG. 3). Has analyzed 611cm-1And 1650cm-1As the main characteristic peak (D in fig. 3), the results showed that the intensity of the characteristic peak of R6G was pulsed with the pulseThe number of impulses increases and monotonically decreases. As the number of pulses increases, the type 2 structure is destroyed, resulting in a decrease in the density of plasma hot spots. Therefore, the raman enhancement effect decreases as the number of pulses increases from 8 to 40. The effect of Ag sputtering time on the enhancement effect was also investigated (E, F in fig. 3). The enhancement effect increases as the sputtering time increases from 100s to 500s, because the diameter of the silver nanoparticles, which have a significant effect on the raman intensity, increases with the increase in the sputtering time.
4. Uniformity of Raman signal
Uniformity of raman signal is equally important as SERS sensitivity, which is crucial for SERS-based substrates in quantitative determination of target analytes. To assess the homogeneity of the Raman signal on MA-SERS, the inventors examined the concentration to be 1X 10- 5The R6G molecule of M signals at 10 points on type 1 and type 2, respectively. All 20 points showed satisfactory signal intensity (a in fig. 4). To further determine the average maximum raman intensity and uniformity of the different regions SERS signal, 611cm was extracted-1And 1650cm-1The data at peak was analyzed (B, C in fig. 4). This indicates that the average maximum raman intensities of the two peaks show similar results and that the raman intensity of type 2 is higher than that of type 1 due to the nanoparticle of type 2 being larger in diameter than that of type 1, as shown in fig. 1D. 611cm of type 2 and type 1-1Has an average maximum Raman intensity of 1263.7 and 1143.9, respectively, and 1650cm-1The average maximum raman intensities of (a) are 2846.6 and 2464.4, respectively. The effect of silver particle size on the electromagnetic field strength was simulated to validate and interpret the results. In the simulation, silver nanospheres with a 2nm fixed spacing between nearest neighboring nanospheres were used, and the diameter of the two nanospheres was changed from 10nm to 100 nm. The electromagnetic field strength increased in the range of 10nm to 80nm nanosphere diameter and decreased in the range of 80-100 nm (FIG. 8). The Relative Standard Deviation (RSD) of the raman intensities in the two regions were calculated separately to further explore the uniformity of these SERS signals. Maximum Raman intensity of type 1 and type 2 is 611cm-1RSD of 8.25% and 9.73%, respectively, and 1650cm-1The maximum raman intensities of (a) and (b) were 11.5% and 11.68%, respectively. The results show that RSD for signals from either type 1 or type 2 are less than 15% each, showing excellent uniformity.
In conclusion, the MA-SERS with the layered micro/nano structure can be prepared by femtosecond laser treatment and magnetron sputtering. By optimizing process parameters and adjusting the layered composite structure, good wettability can be ensured, the contact angle is about 150 degrees, and simultaneously the enhancement factor of Surface Enhanced Raman Scattering (SERS) can reach 1.3 multiplied by 107Sufficient to detect molecular levels. MA-SERS exhibits superior performance and a simple and cost-effective manufacturing process compared to substrates prepared by a common top-down photolithography process. The efficient and economical MA-SERS formulations of the present invention can be used in ultra-low concentration solutions (e.g., as low as 10 concentration) containing, for example, biopharmaceuticals, pesticide residues, and environmental pollutants-10Contaminants of M) has broad application prospects in detection.
Although the present invention has been described above in connection with the exemplary embodiments and the accompanying drawings, it will be apparent to those of ordinary skill in the art that various modifications may be made to the above-described embodiments without departing from the spirit and scope of the claims.
Claims (6)
1. A method for preparing a surface enhanced raman substrate having a layered micro/nano structure, comprising the steps of:
utilizing nanosecond-femtosecond laser pulse to inching and irradiate the surface of a silicon substrate or a silicon dioxide substrate to form a micro-pit array, wherein the diameter of each micro-pit in the micro-pit array is 10-20 mu m, the distance between the outer edges of any two adjacent micro-pits is not more than 1/6 of the diameter, and the micro-pit array is provided with a plurality of nano convex points with the grain size of 30-100 nm;
cleaning and drying the silicon substrate or the silicon dioxide substrate with the micro-pit array;
then, forming a silver film with the thickness of 40-300 nm in a silicon substrate or a silicon dioxide substrate in a magnetron sputtering deposition mode in an area with a micro-pit array to obtain a surface enhanced Raman substrate with a layered micro/nano structure;
the silver film is deposited on the surface of the micro-pit array and can wrap the nano-convex points forming part of the surface of the micro-pit array, the nano-particles formed by the nano-convex points are wrapped by the silver, and the inner core of the nano-particles is silicon or the outer surface of silicon dioxide is a silver film.
2. The method for preparing a surface-enhanced Raman substrate with a layered micro/nano structure according to claim 1, wherein the number of pulses of the laser pulse of nanosecond to femtosecond corresponding to each micro pit in the micro pit array is selected in a range of 8 to 40.
3. The method for preparing a surface-enhanced Raman substrate with a layered micro/nano structure according to claim 2, wherein the power of the laser pulse from nanosecond to femtosecond is 3-10 mW.
4. The method for preparing a surface-enhanced Raman substrate with a layered micro/nano structure according to claim 1, wherein each micro pit in the micro pit array has a diameter of 13-17 μm, the distance between the outer edges of any two adjacent micro pits is from zero to 1/10 of the diameter, and the particle size of the nano convex points is from 65-85 nm.
5. The method for preparing the surface-enhanced Raman substrate with the layered micro/nano structure according to claim 1, wherein the thickness of the silver thin film is 100-180 nm.
6. The method for preparing a surface-enhanced Raman substrate having a layered micro/nano structure according to claim 1, wherein the cleaning step is performed by an organic cleaning agent and ultra-pure water, respectively.
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