Ultrafast Laser-Ablated Nanoparticles and Nanostructures for Surface-Enhanced Raman Scattering-Based Sensing Applications

Sree Satya Bharati Moram; Jagannath Rathod; Dipanjan Banerjee; Venugopal Rao Soma

doi:10.3791/65450

JoVE Journal > Engineering

Engineering

Ultrafast Laser-Ablated Nanoparticles and Nanostructures for Surface-Enhanced Raman Scattering-Based Sensing Applications

Published: June 16, 2023

doi:

10.3791/65450

Sree Satya Bharati Moram¹, Jagannath Rathod¹, Dipanjan Banerjee¹, Venugopal Rao Soma¹

¹Advanced Centre for Research in High Energy Materials (ACRHEM), DRDO Industry Academia – Centre of Excellence (DIA-COE),University of Hyderabad

Summary

Ultrafast laser ablation in liquid is a precise and versatile technique for synthesizing nanomaterials (nanoparticles [NPs] and nanostructures [NSs]) in liquid/air environments. The laser-ablated nanomaterials can be functionalized with Raman-active molecules to enhance the Raman signal of analytes placed on or near the NSs/NPs.

Abstract

The technique of ultrafast laser ablation in liquids has evolved and matured over the past decade, with several impending applications in various fields such as sensing, catalysis, and medicine. The exceptional feature of this technique is the formation of nanoparticles (colloids) and nanostructures (solids) in a single experiment with ultrashort laser pulses. We have been working on this technique for the past few years, investigating its potential using the surface-enhanced Raman scattering (SERS) technique in hazardous materials sensing applications. Ultrafast laser-ablated substrates (solids and colloids) could detect several analyte molecules at the trace levels/mixture form, including dyes, explosives, pesticides, and biomolecules. Here, we present some of the results achieved using the targets of Ag, Au, Ag-Au, and Si. We have optimized the nanostructures (NSs) and nanoparticles (NPs) obtained (in liquids and air) using different pulse durations, wavelengths, energies, pulse shapes, and writing geometries. Thus, various NSs and NPs were tested for their efficiency in sensing numerous analyte molecules using a simple, portable Raman spectrometer. This methodology, once optimized, paves the way for on-field sensing applications. We discuss the protocols in (a) synthesizing the NPs/NSs via laser ablation, (b) characterization of NPs/NSs, and (c) their utilization in the SERS-based sensing studies.

Introduction

Ultrafast laser ablation is a rapidly evolving field of laser-material interactions. High-intensity laser pulses with pulse durations in the femtosecond (fs) to picosecond (ps) range are used to generate precise material ablation. Compared to nanosecond (ns) laser pulses, ps laser pulses can ablate materials with higher precision and accuracy due to their shorter pulse duration. They can generate less collateral damage, debris, and contamination of the ablated material due to fewer thermal effects. However, ps lasers are typically more expensive than ns lasers and need specialized expertise for operation and maintenance. The ultrafast laser pulses enable precise control over the energy deposition, which leads to highly localized and minimized thermal damage to the surrounding material. Additionally, ultrafast laser ablation can lead to the generation of unique nanomaterials (i.e., surfactants/capping agents are not obligatory during the production of nanomaterials). Therefore, we can term this a green synthesis/fabrication method¹^,²^,³. The mechanisms of ultrafast laser ablation are intricate. The technique involves different physical processes, such as (a) electronic excitation, (b) ionization, and (c) the generation of a dense plasma, which results in the ejection of material from the surface⁴. Laser ablation is a simple single-step process to produce nanoparticles (NPs) with high yield, narrow size distribution, and nanostructures (NSs). Naser et al.⁵ conducted a detailed review of the factors influencing the synthesis and production of NPs through the laser ablation method. The review covered various aspects, such as the parameters of a laser pulse, focusing conditions, and the ablation medium. The review also discussed their impact on producing a wide range of NPs using the laser ablation in liquid (LAL) method. The laser-ablated nanomaterials are promising materials, with applications in various fields such as catalysis, electronics, sensing, and biomedical, water splitting applications⁶^,⁷^,⁸^,⁹^,¹⁰^,¹¹^,¹²^,¹³^,¹⁴.

Surface-enhanced Raman scattering (SERS) is a powerful analytical sensing technique that significantly enhances the Raman signal from probe/analyte molecules adsorbed onto metallic NSs/NPs. SERS is based on the excitation of surface plasmon resonances in metallic NPs/NSs, which results in a significant rise in the local electromagnetic field near the metallic nano-features. This enhanced field interacts with the molecules adsorbed on the surface, significantly enhancing the Raman signal. This technique has been used to detect various analytes, including dyes, explosives, pesticides, proteins, DNA, and drugs¹⁵^,¹⁶^,¹⁷. In recent years, significant progress has been made in the development of SERS substrates, including the use of differently shaped metallic NPs¹⁸^,¹⁹ (nanorods, nanostars, and nanowires), hybrid NSs²⁰^,²¹ (a combination of the metal with other materials such as Si²²^,²³, GaAs²⁴, Ti²⁵, graphene²⁶, MOS₂²⁷, Fe²⁸, etc.), as well as flexible substrates²⁹^,³⁰ (paper, cloth, nanofiber, etc.). Developing these new strategies in the substrates has opened up new possibilities for using SERS in various real-time applications.

This protocol discusses the fabrication of Ag NPs using a ps laser at different wavelengths and Ag-Au alloy NPs (with different ratios of Ag and Au targets) fabricated using laser ablation technique in distilled water. Additionally, silicon micro/nanostructures are created using an fs laser on silicon in the air. These NPs and NSs are characterized using ultraviolet (UV)-visible absorption, transmission electron microscopy (TEM), X-ray diffraction (XRD), and field emission scanning electron microscopy (FESEM). Furthermore, the preparation of SERS substrates and analyte molecules are discussed, followed by the collection of Raman and SERS spectra of the analyte molecules. Data analysis is performed to determine the enhancement factor, sensitivity, and reproducibility of the laser-ablated NPs/NSs as potential sensors. Additionally, typical SERS studies are discussed, and the SERS performance of hybrid substrates is evaluated. Specifically, it has been found that the promising gold nanostars' SERS sensitivity can be enhanced approximately 21 times by using laser-structured silicon instead of plain surfaces (such as Si/glass) as a base.

Protocol

A typical protocol flowchart of the application of ultrafast ablated NPs or NSs in the trace detection of molecules via SERS is shown in Figure 1A.

1. Synthesizing metal NPs/NSs

NOTE: Depending on the requirement/application, choose the target material, the surrounding liquid, and the laser ablation parameters.
Here:
Target materials: Ag
Surrounding liquid: 10 mL of DI
Laser parameters: 355/532/1064 nm; 30 ps; 10 Hz; 15 mJ
Focusing lens: Plano-convex lens (focal length: 10 cm)
Stage parameters: 0.1 mm/s along the X and Y directions

Sample cleaning before laser ablation
1. Perform ultrasonic cleaning (40 kHz, 50 W, 30 °C) of the target surface using acetone for 15 min, which removes various organic materials, including oils, greases, and waxes.
2. Then, subject the surface to ultrasonic cleaning with ethanol for another 15 min to remove polar contaminants, such as salts and sugars.
3. Finally, clean the surface with deionized water (DW) using ultrasonic cleaning for 15 min to remove any remaining traces of solvents or contaminants from the sample's surface.
  NOTE: These steps will help to eliminate any unwanted impurities that may be present on the surface, ensuring accurate analysis.
Measuring the weight of the sample
1. Measure the weight of the sample before ablation.
2. Perform the laser ablation experiment on the sample.
3. Measure the weight of the sample again after the ablation experiment.
4. By comparing the weight of the sample before and after ablation, estimate the amount of material that was removed during the experiment. This information will be useful in analyzing the properties of the ablated material, such as the concentration and yield of the ablated products.
Adjust the laser parameters
1. Adjust the input laser power such that it is greater than the ablation threshold of the sample. Here, an input power of ~150 mW was used for ps laser ablation of the Ag target.
  NOTE: The threshold refers to the minimum energy per unit area required to heat the target material to the point where it is vaporized and converted into plasma.
2. Combine a polarizer and a half-wave plate to adjust the laser pulse energy. Figure 1B shows the schematic of ultrafast laser ablation.
Laser focus adjustments on to the sample surface
1. Focus the laser beam onto the sample using a focusing lens to ablate the material surface.
2. Adjust the laser's focus on the sample manually using a translation stage in the Z-direction by observing the bright plasma produced and the emanating cracking sound.
  NOTE: To visualize the plasma generated during the laser ablation experiments, the photographs of both configurations are provided in Figure 2A: (i) laser ablation in air and (ii) laser ablation in liquid (LAL).
Different types of focusing
NOTE: Focusing optics can help increase the energy density of the laser (plasma formation) beam at the sample surface, leading to more efficient ablation. Various types of focusing optics can be used, such as plano-convex lenses, axicon³¹, cylindrical lenses, etc.
1. Use focusing optics for focusing the laser beam onto the sample, depending on the specific requirements, like achieving different ablation depths, allowing for better control over the synthesis of NPs/NSs. Figure 2B shows the three focusing conditions used in LAL.
  NOTE: Adjusting the laser focus onto the sample in laser ablation requires certain precautions to ensure safety and accuracy.
2. Check and maintain the equipment used to manipulate the laser focus to ensure it functions correctly.
3. Adjust the laser focus safely and accurately to minimize the risk of injury or damage to equipment.
  NOTE: The choice of the focal length of the lenses depends on the material used for laser ablation, the type of laser (pulse duration, beam size), and also the desired spot size at the sample surface.
Scanning area of the sample
1. Position the sample on the X-Y stages that are connected to an ESP motion controller. The sample is moving perpendicular to the laser propagation direction.
  NOTE: The ESP motion controller is used to perform a raster scan of the sample in the X and Y directions to prevent single-point ablation.
2. Adjust the scan speed (typically 0.1 mm/s for a better yield of metal NPs) and laser processing area to optimize the number of laser pulses that interact with the sample, as this affects the yield of the NPs.
3. To achieve the desired dimensions and prevent single-point ablation, perform laser patterning while scanning the sample during the laser ablation process.
  NOTE: Figure 3A, B illustrates the fs laser ablation setup photograph by engaging Gaussian and Bessel beams, respectively.
Laser ablation in liquid to synthesize metal NPs/NSs
1. Conduct a laser ablation experiment after setting up all the desired requirements. Follow the steps mentioned in steps 1.1-1.6.
2. Make sure to monitor the laser power and other settings to ensure that they remain consistent throughout the experiment.
3. Continuously observe the target material during the laser ablation experiment to ensure that the laser beam remains focused on the desired area.
  NOTE: Figure 3A,B shows the fs laser ablation experimental setups for synthesizing the NPs using a Gaussian beam and an axicon beam, respectively. A plano-convex lens was used for focusing the input pulses. The formation of NPs is evident from the pictures obtained at different times of the experiment. The color of the solution indicates the formation of NPs, and a color change in the solution indicates an increasing yield of the NPs (depicted in Figure 4). Laser safety goggles must be worn when working in the laser lab, using only approved laser safety eyewear for the proper wavelength. Any stray reflection of the high-power laser beam into the eye is extremely dangerous, resulting in irreversible damage. The laser beam should be kept pointing away from all the people in the laser lab. The optical elements in the setup were not disturbed on the optical table. The sample and stages should be monitored while the experiments are being performed.

2. Storage of colloidal NPs/NSs

Store the synthesized NPs in clean glass bottles and store NSs in airtight containers. Place both inside a desiccator.
NOTE: Figure 5 shows colloidal NPs of various colors synthesized through LAL by combining different liquids and targets. Here, Figure 5A,B displays the typical photographs of different colloidal NPs, including (i) metal NPs, Ag, Au, and Cu NPs in various solvents, such as DW and NaCl; (ii) metal alloy NPs, Ag-Au NPs with different compositions, Ag-Cu NPs, and Au-Cu NPs; and (iii) metal-semiconductor alloy NPs, titanium-Au and silicon-Au/Ag NPs. These photographs illustrate the variety of NPs that can be synthesized using colloidal methods and showcase the unique optical properties of metal-semiconductor alloy NPs. Storing colloidal NPs properly is crucial to ensure their stability and maintain their properties. Glass bottles are preferred over plastic or metal containers as they do not react with the NPs. NPs/NSs should be stored in a container with a tight-fitting lid to minimize exposure to air and kept in a dark place that protects them from light.

3. Characterization of laser-ablated NPs/NSs

NOTE: Characterizing metal NSs/NPs is vital for comprehending their properties and ensuring their quality, such as size, shape, composition, etc.

Absorption spectroscopy
NOTE: UV-visible absorption spectroscopy is a well-established technique for characterizing metal NPs. It is considered fast, simple, and noninvasive, making it a valuable tool for determining various properties of NPs. The position of the peaks is related to various properties of the NPs, such as their material composition, size distribution, shape, and the surrounding medium.
1. Sample preparation for UV-visible absorption studies
  1. Prior to recording the spectrum, ensure that the NPs are evenly distributed and suspended in the solution. Fill a sample cuvette with the 3 mL of NP suspension and a reference cuvette filled with the base solvent (in which the NPs are dispersed). Make sure that the cuvettes are clean and free from contaminants.
  2. Collect the absorption data (in the spectral range from 200-900 nm) using a typical step size of 1 nm.
TEM analysis
NOTE: Colloidal NP size and shape were examined by a transmission electron microscope and later analyzed using the software.
1. TEM grid preparation
  1. Using a micropipette, gently dispense approximately 2 µL of the metal NP suspension onto a TEM grid coated with a thin carbon film on top of a thin copper grid. Let the solvent evaporate naturally at room temperature (RT).
    NOTE: For the collection of TEM images, an accelerating voltage of 200 kV and an electron gun current of ~100 µA were used. The micrographs were collected at different magnifications of 2 nm, 5 nm, 10 nm, 20 nm, 50 nm, 100 nm, and 200 nm. TEM analysis was used to find out the size and shape of the NPs.
SEM analysis
NOTE: The surface morphology of the laser-ablated NSs and deposition/composition of the laser-ablated NPs on the bare Si/NSs was examined using FESEM. A typical photograph of a laser-ablated metal/semiconductor/alloy NS sample is shown in Figure 6.
1. SEM sample preparation: For the SEM characterization of NPs, deposit a small droplet of the NP's suspension onto a cleaned silicon wafer, which serves as the sample holder. Then, dry the sample at RT.
2. Use the metal NSs directly for FESEM characterization without further preparation for surface morphology.
  NOTE: For the collection of FESEM images, the electron high voltage was 3-5 kV and the working distance was typically 5-7 mm, at different magnifications of 5,000x, 10,000x, 20,000x, 50,000x, and 100,000x.
XRD analysis
NOTE: XRD is a commonly used technique for characterizing the crystal structure and crystal quality of NPs.
1. XRD sample preparation
  1. Drop cast 50-100 µL of the NP suspension onto a glass slide. Carefully add the drops to the center of a glass sample drop by drop. Add the drops slowly on the same spot to ensure the NPs are distributed on the glass to obtain good-quality XRD data.
    NOTE: The data was collected from 3°-90° with a step size of 0.01° over a duration of ~1 h. The X-ray wavelength used was 1.54 A°, the generator voltage was 40 kV, and the tube current was 30 mA.
  2. Subsequently dry the sample at RT to obtain a homogeneous, thin film of NPs.
2. XRD data analysis
  1. Analyze the XRD peak positions with Joint Committee on Powder Diffraction Standards (JCPDS) Cards. Each JCPDS card contains information on a specific material's crystal structure, lattice parameters, and XRD pattern.

4. Application of the NPs/NSs

Raman analysis
1. Initially, collect the desired analyte molecules' Raman spectra in powder form. Analyze the collected Raman data to identify the spectral peaks corresponding to the vibrational modes of the analyte molecule.
Stock solution preparation
1. Confirm the solubility of the analyte molecules in the chosen solvent. Then, prepare stock solutions of the analyte molecules with accurately weighed or measured amounts.
2. For example, to prepare a 50 mM stock solution of methylene blue (MB) molecule in 5 mL of ethanol:
  1. Calculate the amount of MB powder needed using the formula: mass = concentration (in mM) x volume (in L) x molecular weight (in g/mol). In this case, mass = 50 mM x 0.005 L x 319 g/mol = 0.7995 g or approximately 800 mg.
  2. Weigh out 800 mg of MB powder using a digital balance. Add the powder to a clean glass bottle.
  3. Add solvent to the bottle and shake vigorously to dissolve the powder. Tightly seal the bottle cap and mix the solution thoroughly.
Raman data collection
1. Collect the stock solution Raman spectra by depositing a 10 µL drop of stock solution on a piece of clean silicon wafer. Figure 7A shows the photograph of a portable Raman spectrometer with a 785 nm laser excitation.
Analyte molecule preparation
1. Using a micropipette, dilute the stock solution to different concentrations by adding an appropriate volume of solvent to a series of glass vials depending on the concentration range of interest.
2. Prepare the series of dilutions from a 50 mM stock solution to a final concentration using the formula C_known x V_known = C_unknown x V_unknown.
SERS substrate preparation
1. To prepare a SERS substrate using NPs, deposit a small drop of NPs on a clean silicon surface and allow it to dry. Then, place a tiny drop of the desired analyte molecule on the NP-coated silicon substrate. A schematic of the preparation of SERS substrates using NPs, hybrid, and metal NSs is shown in Figure 7B.
SERS spectra collection
1. Collect the SERS data using a portable Raman spectrometer with a 785 nm laser excitation source. Compare the Raman peaks of the analyte molecule to the spectra with those of reference standards (powder and stock solution).
SERS data analysis
1. Process the obtained Raman and SERS spectra for background correction, subtraction of fluorescence signals, smoothing of the signal, and baseline correction.
  1. Import the text file into ORIGIN software and then follow the steps: analysis > peak and baseline > peak analyser > open dialogue > substract baseline > next > user defined > add base line correction point > done > finish.
    NOTE: One can write their own Matlab/Python program to achieve this.
2. Analyze the resulting peaks in terms of their positions and intensities by placing the reader/annotation point on the peak (in ORIGIN).
3. Assign the peaks to their corresponding Raman vibrational mode assignments based on their spectral characteristics by collecting the bulk Raman spectrum, literature survey, and/or density functional theory (DFT) calculations.
Sensitivity calculation
1. Calculate the enhancement factor (EF) scale, defined as the ratio of the Raman signal intensity obtained from the SERS active substrate to that obtained from the non-plasmonic substrate for a specific Raman mode of the analyte molecule.
Limit of detection
1. Perform quantitative SERS analysis using a linear calibration curve, which represents the relationship between the concentration of the target analyte and its measured Raman signal intensity.
  Limit of detection (LOD) = 3 x (standard deviation of the background noise)/(slope of the calibration curve).
Reproducibility
NOTE: The ability of the substrate to consistently produce the same or similar SERS signals for a given analyte molecule under the same experimental conditions is referred to as the reproducibility of the SERS substrate.
1. Calculate the relative standard deviation (RSD) as follows: RSD = (standard deviation/mean) x 100%
  NOTE: In general, RSD values in the 5%-20% range are considered acceptable for most SERS experiments, but lower RSD values are often desirable for more quantitative and reliable SERS measurements

Representative Results

Silver NPs were synthesized via ps laser ablation in liquid technique. Here, a ps laser system with a pulse duration of ~30 ps operating at a 10 Hz repetition rate and with a wavelength of one of 355, 532, or 1,064 nm was used. The input pulse energy was adjusted to 15 mJ. The laser pulses were focused using a plano-convex lens with a focal length of 10 cm. The laser focus should be exactly on the material surface during laser ablation because the laser energy is most concentrated at the focal point, where it can cause the desired material removal. If the laser focus is not on the material surface, the laser energy is distributed over a larger area; it may not be sufficient for material removal or surface modifications. It may finally lead to an incomplete or inconsistent ablation. The sample was translated using 0.1 mm/s along the X and Y directions. The Ag target was immersed in 10 mL of DI, and the liquid height above the sample was ~7 mm. In general, the height of the solvent should be sufficient to cover the entire target material and prevent the material from overheating during laser ablation. Also, if the liquid height is too high, it can absorb some of the input laser energy before it reaches the target material, leading to a reduced ablation mechanism and a lower yield of NPs. If the liquid height is too low, at higher input laser energies, it may lead to agglomeration of the NPs. Additionally, it should be chosen to provide sufficient dispersion of the ablated material and prevent agglomeration of the NPs. The weight of the target is measured before and after the ablation process will give an idea of the amount of material that has been removed. Here, the ablated mass was estimated to be ~0.37, ~0.38, and ~0.41 mg at 355, 532, and 1,064 nm, respectively. This is important for estimating the yield of the desired colloidal NPs and ensuring the process is reproducible under the same experimental conditions. Next, the synthesized Ag NPs were characterized by UV-visible absorption spectroscopy. This method measures the amount of light absorbed by the NPs at different wavelengths in the UV-visible near infrared (NIR) regions of the spectrum. The absorption spectra obtained from UV-visible spectroscopy can be used to determine the localized surface plasmon resonance (LSPR) of the NPs. LSPR is a collective oscillation of electrons in the NPs, resulting in an absorption peak in the UV-visible region.

Figure 8A shows the absorption spectra of Ag colloidal NPs achieved by ps laser ablation of Ag in DW at different wavelengths (355 nm, 532 nm, and 1,064 nm). The spectra reveal that the surface plasmon resonance (SPR) peaks of the resulting NPs were located at ~420 nm, ~394 nm, and ~403 nm for the NPs attained at 355 nm, 532 nm, and 1,064 nm, respectively. The absorbance of the NPs increased with decreasing laser wavelength. This may be attributed to the higher levels of self-absorption of the laser pulses at lower wavelengths. Figure 8B illustrates the normalized absorbance spectra of Ag-Au alloy NPs with different compositions. The SPR peak position shifted from 410 nm to 519 nm, with an increase in the Au percentage from 0% to 100%. Figure 8C represents a correlation between the SPR peak position and Au mole fraction in Ag-Au alloy NPs. This relationship provides a useful tool for predicting the SPR peak position of Ag-Au alloy NPs with different compositions, which can aid in the design and synthesis of NPs with specific optical properties. Further, TEM studies were performed to examine the size and shape of Ag NPs. Figure 9A–C presents the TEM images of Ag NPs in DW at 355 nm, 532 nm, and 1,064 nm, respectively. The shape of Ag NPs was spherical, and the size distribution of Ag NPs in DW is shown in Figure 9D–F. The average sizes of the Ag NPs were ~12.4 nm ± 0.27 nm, ~23.9 nm ± 1.0 nm, and ~25 nm ± 0.7 nm, respectively, at 355 nm, 532 nm, and 1,064 nm. The average size of Ag NPs fabricated with 1,064 nm laser light was larger than that of those fabricated with 355 nm and 532 nm laser pulses. It has been reported that the increase in NP sizes with increasing wavelength was presumably the coexistence of LAL and the self-absorption of laser light by colloid metal particles causing laser fragmentation in liquids (LFL). Further, typical XRD patterns of Ag NPs on glass slides were recorded (Figure 10). The 2theta positions refer to the angles at which a crystalline material diffracts the X-rays. The angle is measured between the incident X-ray beam and the detector and is expressed in degrees. The peak maxima are positioned at 38.4°, 44.6°, 64.7°, and 77.7°, and correspond to the Bragg reflections of Ag from the planes having miller indices (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2), respectively. The noticed peaks are matched with JCPDS file number Ag: 03-0921 with a face-centered cubic structure.

Further, the typical FESEM micrographs of Ag NP-deposited Si, single-line laser-ablated Si, cross-patterned on Si, and laser-ablated iron (Fe) in acetone morphologies are provided, which are shown in Figure 11. Depending on the laser-material interaction, the morphology of the substrate structures, such as LIPSS/Groove/ripples, etc., can be formed. Typical FESEM images of star-shaped Au NPs deposited on the bare Si and laser-ablated cross-patterned Si surfaces are depicted in Figure 12. The distribution of the Au NPs on bare Si is shown in Figure 12A. Figure 12B–D presents the distribution of Au nanostars on the laser-ablated Si surface. Figure 12B shows the distribution on the non-interacted surface, while Figure 12C,D illustrates the FESEM images of laser-patterned Si micro/nanostructures with the Au NPs distribution.

Afterward, the application of the laser-ablated NPs/NSs in SERS studies were executed. The Raman and SERS substrate preparation (with and without NPs) and the collection of the corresponding Raman and SERS spectra of MB are shown in Figure 13. The enhancement of the Raman peaks of the MB molecule was clearly observed even at a concentration of 5 µM, which is 20,000 times less than the concentration used for Raman – 100 mM (stock solution). The MB molecule Raman peaks were enhanced in the presence of NPs compared to those without NPs. Figure 14A–C illustrates the obtained SERS intensity of MB, NB, and thiram using the laser-patterned silicon (at different pulse numbers by varying scan speed and pattern): (i) Si_5L, (ii) Si_5C, (iii) Si_0.5L, and (iv) Si_0.5C with Au nanostars. The Raman enhancement of the three molecules is noticed from the Si NSs, and also the reproducibility is verified from 15 different locations from four substrates. The histogram plot in Figure 14D with the RSD for Si_5L, Si_5C, Si_0.5L, and Si_0.5C reveals that the Si NSs with star substrates showed a better SERS signal all over the area.

Figure 1: Schematic and flowchart of ultrafast laser ablation. (A) Typical flow chart of trace detection using ultrafast laser-ablated NPs/NSs via SERS. (B) Schematic of ultrafast laser ablation in liquid. Please click here to view a larger version of this figure.

Figure 2: Photographs of various laser ablation experiments. (A) Photographs of (a) LAL of Au in air and (b) LAL of Ag target in a gold salt (HAuCl₄) solution (bright spot shown is the plasma). (B) Photographs of laser ablation in different focusing conditions with a (a) plano-convex lens, (b) axicon lens, and (c) cylindrical lens. Here, there is typically a 7 mm liquid height for 10 mL of solution at 500 µJ for the plano-convex lens, 3 mm height for 5 mL of solution for the Bessel beam, and 10 mm height for 10 mL for the cylindrical lens. Please click here to view a larger version of this figure.

Figure 3: Photographs of fs laser ablation. (A) Photograph of (a) the fs laser ablation setup and the resulting (b) metal NPs (during laser ablation), and (c) metal NSs (after laser ablation) using the plano-convex lens. (B) Photograph of (a) fs laser ablation using the axicon lens and a (b) zoomed-in image of the photograph. Please click here to view a larger version of this figure.

Figure 4: Photographs of fs laser ablation in liquid at different times using the plano-convex lens. (A) 1 min, (B) 5 min, (C) 20 min. Please click here to view a larger version of this figure.

Figure 5: Photographs of laser-ablated colloidal NPs. (A) Au NPs at different energies (µJ): (a) 200, (b) 300, (c) 400, and (d) 500 by fs laser ablation in DW. (B) (a) Ag NPs, (b) Au NPs, and (c) Cu NPs by fs LAL in DW. (C) Aggregated Au NPs (fs laser ablation in DW) in different concentrations of NaCl (mM): (a) 1, (b) 10, (c) 50, (d) 100, (e) 500 mM, and (f) 1 M³². (D) (a) Pure Ag, (b) Ag₅₀Au₅₀,and (c) pure Au by ps laser ablation in NaCl. (E) Alloy NPs: (a) pure Ag, (b) Ag₇₀Au₃₀, (c) Ag₅₀Au₅₀, (d) Ag₃₀Au₇₀, and (e) pure Au by ps laser ablation in distilled water colloidal NPs. (F) Alloy NPs: (a) Ag₆₀Au₄₀, (b) Ag₅₀Au₅₀, (c) Ag₄₀Au₆₀, (d) Ag₃₀Au₇₀, and (e) Ag₂₀Au₈₀ by fs laser ablation in acetone. (G) Metal alloy NPs: (a) Cu_Au, (b) Ag_Au, and (c) Ag_Cu. (H) Metal semiconductor alloy NPs: (a) Au_TiO₂, (b) Ag-SiO₂, and (c) Au_SiO₂ NPs Please click here to view a larger version of this figure.

Figure 6: Photograph of fs laser-ablated NPs. (A) Ag, (B) Au, (C) Cu, (D) Si, and (E–H) Ag-Au alloy NPs with different ratios of Ag and Au. Please click here to view a larger version of this figure.

Figure 7: SERS substrate preparation. (A) Photograph of the portable Raman spectrometer. (B) Schematic of the SERS substrate preparation using (1) colloidal metal NPs, (2) rigid metal NSs, and (3) hybrid substrate (NSs+NPs). Please click here to view a larger version of this figure.

Figure 8: Absorption spectra. (A) Absorption spectra of the ps laser-ablated Ag NPs in DW using different laser wavelengths. (B) Normalized UV-visible absorption spectra of ps laser (1064 nm)-ablated Au-Ag NPs: (i) pure Ag, (ii) Ag₇₀Au₃₀, (iii) Ag₅₀Au₅₀, (iv) Ag₃₀Au₇₀, and (v) pure Au. (C) Shift in the SPR peak position with increasing Au percentage in Ag-Au alloy NPs. Panels B and C have been reproduced with permission from Byram et al³³. Please click here to view a larger version of this figure.

Figure 9: TEM images and their respective size histograms of Ag NPs fabricated in DW using 30 ps laser pulses. (A,D) 355 nm, (B,E) 532 nm, and (C,F) 1,064 nm. Please click here to view a larger version of this figure.

Figure 10: XRD pattern of ps (1,064 nm) laser-ablated Ag NPs in DW. Please click here to view a larger version of this figure.

Figure 11: FESEM images. (A) Ag-Au alloy NPs deposited Si. (B) Single line ablation of Si. (C) Cross-patterned ablation on Si. (D) Fe NSs in acetone using fs laser ablation. Please click here to view a larger version of this figure.

Figure 12: FESEM images. (A) Au nanostars on the bare Si. (B–D) Au NPs decorated on different areas of laser-ablated Si: (B) area of unprocessed Si with redeposited Si NPs, (C) within the channel written using laser pulses, and (D) at the edge of the channel with spikes. This figure has been reproduced with permission from Moram et al.³⁴. Please click here to view a larger version of this figure.

Figure 13: Raman and SERS spectra of the MB molecule. Schematic of Raman and SERS spectra collection with preparation and the typical obtained Raman (MB: 100 mM, red color) and SERS (5 µM, green colors) spectra of the MB molecule Please click here to view a larger version of this figure.

Figure 14: SERS spectra. (A) MB: 1.6 ppb, (B) NB: 1.8 ppb, and (C) thiram: 0.1 ppm using star-shaped Au NPs on linear and cross-patterned Si using fs laser ablation in the air at different scan speeds-5 mm/s and 0.5 mm/s: (i) plain Si, (ii) Si_5 mm/s -Linear, (iii) Si_5 mm/s-crossed, (iv) Si_0.5 mm/s-linear, and (v) Si_0.5 mm/s-crossed. MB, NB, and thiram molecular structures are also shown as an inset of the figures. (D) Histogram of prominent peak intensity variation from 15 random sites from all four Si substrates with Au nanostars. This figure has been reproduced with permission from Moram et al.³⁴. Please click here to view a larger version of this figure.

Discussion

In ultrasonication cleaning, the material to be cleaned is immersed in a liquid and high-frequency sound waves are applied to the liquid using an ultrasonic cleaner. The sound waves cause the formation and implosion of tiny bubbles in the liquid, generating intense local energy and pressure that dislodge and remove dirt and other contaminants from the surface of the material. In laser ablation, a Brewster polarizer and a half-wave plate combination were used to tune the laser energy; the polarizer is typically placed before the half-wave plate. The polarizer, which is mounted on a rotating mount, allows only light waves of a specific polarization to pass through while reflecting light waves of a perpendicular polarization. The light that passes through the polarizer then enters the half-wave plate, which rotates the polarization of the transmitted light by 90°. When the sample was ablated in the air, only NSs were formed. However, when the sample was securely attached to the bottom of a clean glass beaker, filled with the intended volume of liquid, and ablated in liquid, both NPs and NSs were formed. The portion of the sample that was ablated by the laser contains NSs, while the ablated material dispersed in the surrounding liquid consists of NPs. LAL is a process in which ultrashort laser pulses are directed toward a sample submerged in a liquid, causing localized vaporization of the material. This results in the formation of NPs and NSs in a single step.

LAL has several advantages over other NPs synthesis methods. It is fast, efficient, scalable, and surfactant free. Additionally, the choice of solvent, the concentration of the target material in the solvent, and the presence of any surfactants or stabilizing agents can also impact the NP synthesis process, and therefore should be carefully considered and controlled. The processing and laser parameters (laser fluence, wavelength, pulse duration, repetition rate) can be adjusted to control the size, shape, composition, and surface properties of the produced NPs. Depending on the material, the penetration depth and the material's ablation threshold depend on the incident laser wavelength. All parameters will affect the yield of NPs/morphology of NSs. This level of control allows for tailoring the properties of nanomaterials to meet the specific requirements of diverse applications. The color of metal NPs is a primary and simple indication of their size and shape, as well as the material they are made of³. When light interacts with metal NPs, the electrons in the metal absorb and re-emit the light at specific wavelengths, leading to the color observed. The LAL technique uses bulk targets, which are cheaper than salts used in the wet-chemical technique. Moreover, hazardous waste is generated during the chemical process. Although the wet-chemical technique has a lower initial investment cost compared to the LAL technique, the latter requires a higher initial investment. However, the cost of LAL gradually decreases over time and ultimately becomes cheaper due to the lower cost of reactants². Currently, many companies worldwide have launched startups focused on commercializing products synthesized using laser technology. Examples include IMRA (USA), Particular GmbH (Germany), and Zhongke Napu New Materials Co. Ltd. (China)³⁵.

Recently, a lot of studies have been conducted to achieve superior SERS substrates using ultrafast laser techniques. Yu et al.⁸ have recently reported a hybrid super-hydrophobic/hydrophilic SERS platform by fs laser ablation and detected R6G with an EF of ~10¹³. Dipanjan et al. have reported the formation of ladder-like laser-induced periodic surface structures (LIPSS) on Ag-Au-Cu using fs Bessel beam ablation and successfully detected two explosive (tetryl and pentaerythritol tetranitrate) traces (200 nM)³¹. Verma et al. have used the technique of LAL and fabricated Au-Pd core@shell NPs by LAL, and used them in explosive (PA -10^-7and AN- 10^-8) trace detection³⁶. Verma et al. have again utilized Au NPs deposited on laser-textured Sn and detected PA at a concentration of 0.37 µM and AN at 2.93 nM³⁷.

During SERS measurements, when a small volume of NPs is dropped onto a substrate and left to dry, a spontaneous hydrodynamic process occurs, creating a local flow field within the drop. This flow carries the NPs to the edge of the drop, resulting in a phenomenon known as the "coffee ring" effect, where a dense array of NPs accumulates at the edge of the drop, rather than being evenly distributed throughout. While this natural process can increase the number of hot spots, it may also affect the reproducibility of the SERS signals⁸. The deposition of NPs onto the substrate depends on the contact angle between the solvent and the surface. The wetting behavior of the substrate can be altered by tuning the laser processing parameters in the laser ablation technique. Mangababu et al.²⁴ have shown that the contact angle of GaAs laser ablation can vary in different surrounding liquids, such as distilled water, ethanol, and polyvinyl alcohol. Another possible way to avoid the coffee ring effect is to heat the substrate to 70 °C, for instance, and then drop cast the analyte so that it dries up very fast.

The EF is an important factor for characterizing the performance of the SERS active substrate, and it depends on various factors, such as the morphology of the substrate, molecular geometry of the analyte, excitation wavelength, and polarization of the excitation laser. The EF also depends on the orientation of the analyte molecule with respect to the local field, the orientation of the substrate with reference to the incident laser direction, and the thickness of the analyte layer on the substrate. The EF is estimated using the simple relation EF = (I_SERS x I_R)/(C_R x C_SERS), where I_SERS is the Raman signal intensity with NPs on Si/FP, I_R is the Raman intensity on Si/FP (without NPs), C_SERS is the concentration of the sample on NP substrates (low concentration), and C_R is the concentration of the sample (0.1 M) which produces the Raman signal (I_R)³⁰^,³²^,³⁴. A series of standards with known concentrations of the analyte molecule is measured, and the Raman signal intensity of the most prominent peak is plotted against the concentration. The slope of the resulting line represents the sensitivity of the SERS measurement, and the intercept represents the background signal. The limit of detection (LOD), which is the smallest concentration of the target analyte that can be reliably detected, is estimated from the linear calibration curve. From this, we can estimate the sensitivity of the prepared SERS substrate. Multiple SERS measurements were performed on the same substrate in different locations and the intensity values of the most prominent peak were noted. RSD is a commonly used metric to characterize the reproducibility and reliability of SERS signals. It is defined as the standard deviation (SD) ratio of a set of measurements to the mean value expressed as a percentage. The RSD is a measure of the variability of the SERS signals, and it provides information about the precision of the measurements. A low RSD value indicates high precision and reproducibility, while a high RSD value indicates low precision and high variability³⁰^,³⁴.

Producing star-shaped NPs using LAL is challenging, but they are proven to be superior SERS substrates due to the multiple hot spots arising from the strong electromagnetic fields at the sharp edges/tips¹⁹. Most studies have reported differently shaped metal NP deposition on plain Si/glass³⁸^,³⁹. Here, we have shown further improvement in the sensitivity of metal NPs by using laser-textured Si instead of a plain Si surface. The hybrid SERS substrates, consisting of laser-ablated Si NSs and chemically synthesized Au nanostars, exhibited ~21 times the enhancement of the SERS signal compared to plain Si. Even with our laser-synthesized metal NPs, better SERS performance can be achieved by depositing them on laser-textured material. Previously, we demonstrated that laser-ablated Ag NPs coupled with laser-ablated Ag NSs for detecting 2, 4-dinitrotoluene provided one order of increment in the EF⁴⁰. Here, we aimed to demonstrate that laser-ablated NSs can be used as a platform for any size/shape NPs to achieve better sensitivity and reproducibility. We firmly believe there is tremendous scope for ultrafast laser-ablated NPs and NSs in SERS-based sensing applications²^,³⁸^,³⁹^,⁴¹^,⁴²^,⁴³.

Disclosures

The authors have nothing to disclose.

Acknowledgements

We thank the University of Hyderabad for support through the Institute of Eminence (IoE) project UOH/IOE/RC1/RC1-2016. The IoE grant obtained vide notification F11/9/2019-U3(A) from the MHRD, India. DRDO, India is acknowledged for funding support through ACRHEM [[#ERIP/ER/1501138/M/01/319/D(R&D)]. We acknowledge the School of Physics, UoH, for the FESEM characterization and XRD facilities. We would like to extend our sincere gratitude to Prof SVS Nageswara Rao and his group for their valuable collaboration contributions and support. We would like to express our appreciation to past and present lab members Dr. P Gopala Krishna, Dr. Hamad Syed, Dr. Chandu Byram, Mr. S Sampath Kumar, Ms. Ch Bindu Madhuri, Ms. Reshma Beeram, Mr. A Mangababu, and Mr. K Ravi Kumar for their invaluable support and assistance during and after the laser ablation experiments in the lab. We acknowledge the successful collaboration of Dr. Prabhat Kumar Dwivedi, IIT Kanpur.

Materials

Alloys	Local goldsmith	N/A	99% pure
Axicon	Thorlabs	N/A	100, IR range, AR coated, AX1210-B
Ethanol	Supelco, India	CAS No. 64-17-5
Femtosecond laser	femtosecond (fs) laser amplifier Libra HE, Coherent	N/A	Pulse duraction 50 fs; wavelenngth 800 nm; Rep rate 1 KHz; Pulse Energy: 4 mJ
FESEM	Carl ZEISS, Ultra 55	N/A
Gatan DM3	www.gatan.com	Gatan Microscopy Suite 3.x
Gold target	Sigma-Aldrich, India		99% pure
HAuCl4.3H2O	Sigma-Aldrich, India	CAS No. 16961-25-4
High resolution translational stages	Newport SPECTRA PHYSICS GMBI	N/A	M-443 High-Performance Low-Profile Ball Bearing Linear Stage; The stage is only 1 inch high, and has 2 inches of travel.
Micro Raman	Horiba LabRAM	N/A	Grating-1,800 and 600 grooves/mm; Wavelength of excitation-785 nm,632 nm, 532 nm, 325 nm; Objectives 10x, 20x, 50 x, 100x; CCD detector
Mirrors	Edmund Optics	N/A	Suitable mirrors for specific wavelength of laser
Motion controller	NEWPORT SPECTRA PIYSICS GMBI	N/A	ESP300 Controller-3 axes control
Origin	www.originlab.com	Origin 2018
Picosecond laser	EKSPLA 2251	N/A	Pulse duraction 30ps; wavelenngth 1064 nm, 532 nm, 355 nm; Rep rate 10 Hz; Pulse Energy: 1.5 to 30 mJ
Planoconvex lens		N/A	focal length 10 cm
Raman portable	i-Raman plus, B&W Tek, USA	N/A	785 nm, ~ 100 µm laser spot fiber optic probe excitation and collection
Silicon wafer	Macwin India Ltd.		1–10 Ω-cm, p (100)-type
Silver salt (AgNO₃)	Finar, India	CAS No. 7783-90-6
Silver target	Sigma-Aldrich, India	CAS NO 7440-22-4	99% pure
TEM	Tecnai TEM	N/A
TEM grids	Sigma-Aldrich, India	TEM-CF200CU	Copper Grid Carbon Coated 200 mesh
Thiram	Sigma-Aldrich, India	CAS No. 137-26-8
UV	Jasco V-670	N/A
XRD	Bruker D8 advance	N/A

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Moram, S. S. B., Rathod, J., Banerjee, D., Soma, V. R. Ultrafast Laser-Ablated Nanoparticles and Nanostructures for Surface-Enhanced Raman Scattering-Based Sensing Applications. J. Vis. Exp. (196), e65450, doi:10.3791/65450 (2023).

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